Enriched housing differentially alters allostatic load and cardiopulmonary responses to wildfire-related smoke in male and female mice.

Living conditions are an important modifier of individual health outcomes and may lead to higher allostatic load (AL). However, housing-induced cardiovascular and immune effects contributing to altered environmental responsiveness remain understudied. This investigation was conducted to examine the influence of enriched (EH) versus depleted housing (DH) conditions on cardiopulmonary functions, systemic immune responses, and allostatic load in response to a single wildfire smoke (WS) exposure in mice. Male and female C57BL/6J mice were divided into EH or DH for 22 weeks, and cardiopulmonary assessments measured before and after exposures to either one-hr filtered air (FA) or flaming eucalyptus WS exposure. Male and female DH mice exhibited increased heart rate (HR) and left ventricular mass (LVM), as well as reduced stroke volume and end diastolic volume (EDV) one week following exposure to WS. Female DH mice displayed significantly elevated levels of IL-2, IL-17, corticosterone and hemoglobin A1c (HbA1c) following WS, while female in EH mice higher epinephrine levels were detected. Female mice exhibited higher AL than males with DH, which was potentiated post-WS exposure. Thus, DH increased susceptibility to extreme air pollution in a gender-dependent manner suggesting that living conditions need to be evaluated as a modifier of toxicological responses.

Burn pit-related smoke causes developmental and behavioral toxicity in zebrafish: Influence of material type and emissions chemistry.

Combustion of mixed materials during open air burning of refuse or structural fires in the wildland urban interface produces emissions that worsen air quality, contaminate rivers and streams, and cause poor health outcomes including developmental effects. The zebrafish, a freshwater fish, is a useful model for quickly screening the toxicological and developmental effects of agents in such species and elicits biological responses that are often analogous and predictive of responses in mammals. The purpose of this study was to compare the developmental toxicity of smoke derived from the burning of 5 different burn pit-related material types (plywood, cardboard, plastic, a mixture of the three, and the mixture plus diesel fuel as an accelerant) in zebrafish larvae. Larvae were exposed to organic extracts of increasing concentrations of each smoke 6-to-8-hr post fertilization and assessed for morphological and behavioral toxicity at 5 days post fertilization. To examine chemical and biological determinants of toxicity, responses were related to emissions concentrations of polycyclic hydrocarbons (PAH). Emissions from plastic and the mixture containing plastic caused the most pronounced developmental effects, including mortality, impaired swim bladder inflation, pericardial edema, spinal curvature, tail kinks, and/or craniofacial deformities, although all extracts caused concentration-dependent effects. Plywood, by contrast, altered locomotor responsiveness to light changes to the greatest extent. Some morphological and behavioral responses correlated strongly with smoke extract levels of PAHs including 9-fluorenone. Overall, the findings suggest that material type and emissions chemistry impact the severity of zebrafish developmental toxicity responses to burn pit-related smoke.

Depleted housing elicits cardiopulmonary dysfunction after a single flaming eucalyptus wildfire smoke exposure in a sex-specific manner in ApoE knockout mice.

Although it is well established that wildfire smoke exposure can increase cardiovascular morbidity and mortality, the combined effects of non-chemical stressors and wildfire smoke remains understudied. Housing is a non-chemical stressor that is a major determinant of cardiovascular health, however, disparities in neighborhood and social status have exacerbated the cardiovascular health gaps within the United States. Further, pre-existing cardiovascular morbidities, such as atherosclerosis, can worsen the response to wildfire smoke exposures. This represents a potentially hazardous interaction between inadequate housing and stress, cardiovascular morbidities, and worsened responses to wildfire smoke exposures. The purpose of this study was to examine the effects of enriched (EH) versus depleted (DH) housing on pulmonary and cardiovascular responses to a single flaming eucalyptus wildfire smoke (WS) exposure in male and female apolipoprotein E (ApoE) knockout mice, which develop an atherosclerosis-like phenotype. The results of this study show that cardiopulmonary responses to WS exposure occur in a sex-specific manner. EH blunts adverse WS-induced ventilatory responses, specifically an increase in tidal volume (TV), expiratory time (Te), and relaxation time (RT) after a WS exposure, but only in females. EH also blunted a WS-induced increase in isovolumic relaxation time (IVRT) and the myocardial performance index (MPI) 1-wk after exposures, also only in females. Our results suggest that housing alters the cardiovascular response to a single WS exposure, and that DH might cause increased susceptibility to environmental exposures that manifest in altered ventilation patterns and diastolic dysfunction in a sex-specific manner.

Mild allergic airways responses to an environmental mixture increase cardiovascular risk in rats.

Recent epidemiological findings link asthma to adverse cardiovascular responses. Yet, the precise cardiovascular impacts of asthma have been challenging to disentangle from the potential cardiovascular effects caused by asthma medication. The purpose of this study was to determine the impacts of allergic airways disease alone on cardiovascular function in an experimental model. Female Wistar rats were intranasally sensitized and then challenged once per week for 5 weeks with saline vehicle or a mixture of environmental allergens (ragweed, house dust mite, and Aspergillus fumigatus). Ventilatory and cardiovascular function, measured using double-chamber plethysmography and implantable blood pressure (BP) telemetry and cardiovascular ultrasound, respectively, were assessed before sensitization and after single and final allergen challenge. Responses to a single 0.5 ppm ozone exposure and to the cardiac arrhythmogenic agent aconitine were also assessed after final challenge. A single allergen challenge in sensitized rats increased tidal volume and specific airways resistance in response to provocation with methacholine and increased bronchoalveolar lavage fluid (BALF) eosinophils, neutrophils, lymphocytes, cytokines interleukin (IL)-4, IL-5, IL-10, IL-1ß, tumor necrosis factor-α, and keratinocyte chemoattract-growth-related oncogene characteristic of allergic airways responses. Lung responses after final allergen challenge in sensitized rats were diminished, although ozone exposure increased BALF IL-6, IL-13, IL-1 ß, and interferon-γ and modified ventilatory responses only in the allergen group. Final allergen challenge also increased systolic and mean arterial BP, stroke volume, cardiac output, end-diastolic volume, sensitivity to aconitine-induced cardiac arrhythmia, and cardiac gene expression with lesser effects after a single challenge. These findings demonstrate that allergic airways responses may increase cardiovascular risk in part by altering BP and myocardial function and by causing cardiac electrical instability.

Impacts of a perinatal exposure to manganese coupled with maternal stress in rats: Tests of untrained behaviors.

Manganese (Mn), an element that naturally occurs in the environment, has been shown to produce neurotoxic effects on the developing young when levels exceed physiological requirements. To evaluate the effects of this chemical in combination with non-chemical factors pregnant Long-Evans rats were treated with 0, 2, or 4 mg/mL Mn in their drinking water from gestational day (GD) 7 to postnatal day (PND) 22. Half of the dams received a variable stress protocol from GD13 to PND9, that included restraint, small cage with reduced bedding, exposure to predator odor, intermittent intervals of white noise, lights on for 24 h, intermittent intervals of lights on during dark cycle and cages with grid floors and reduced bedding. One male and one female offspring from each litter were tested to assess untrained behavior. Ultrasonic vocalizations (USV) were recorded from PND13 pups while they were isolated from the litter. Locomotor activity (MA) was measured in figure-eight mazes at PND 17, 29, and 79 (different set of rats at each time point). Social approach (SA) was tested at PND48. Acoustic startle response (ASR) and pre-pulse inhibition (PPI) were measured starting at PND58. At PND53 a sweetness preference for a chocolate flavored milk solution was assessed. There were sex related differences on several parameters for the USVs. There was also a Mn by stress by sex interaction with the females from the 4 mg/mL stressed dams having more frequency modulated (FM) call elements than the 4 mg/mL non-stressed group. There was an effect of Mn on motor activity but only at PND29 with the 2 mg/mL group having higher counts than the 0 mg/mL group. The social approach test showed sex differences for both the habituation and test phase. There was an effect of Mn, with the 4 mg/mL males having a greater preference for the stimulus rat than did the 0 mg/mL males. There was also a stress by sex interaction. The ASR and PPI had only a sex effect. Thus, with only the FM call elements having a Mn by stress effect, and the PND29 MA and SA preference index having a Mn effect but at different doses requires further investigation.

Impacts of a perinatal exposure to manganese coupled with maternal stress in rats: Maternal somatic measures and the postnatal growth and development of rat offspring.

Psychological stress experienced by the mother during pregnancy has been associated with emotional and cognitive disorders in children such as depression and anxiety. Socioeconomically disadvantaged populations are vulnerable to adverse life experiences and can also be disproportionally exposed to environmental contaminants. To better understand the neurodevelopmental impacts of an environmental toxicant coupled with elevated psychological stress, we exposed pregnant rats to a series of perinatal stressors. Manganese (Mn), a neurotoxicant at excessive concentrations was delivered through drinking water (0, 2, or 4 mg/mL) from gestational day (GD) 7 to postnatal day (PND) 22. A variable stress paradigm was applied to half of the animals from GD13 to PND9. Measurements of somatic development and behavior were examined in the offspring at different developmental stages. No evidence of overt maternal toxicity was observed although the 4 mg/mL Mn-exposed dams gained less body weight during gestation compared to the other dams. Stress also reduced gestational maternal weight gain. Daily fluid consumption normalized for body weight was decreased in the Mn-exposed dams in a dose-dependent manner but was not altered by the stress paradigm. Maternal stress and/or Mn exposure did not affect litter size or viability, but pup weight was significantly reduced in the 4 mg/mL Mn-exposed groups on PNDs 9 through 34 when compared to the other offspring groups. The efficacy of the manipulations to increase maternal stress levels was determined using serum corticosterone as a biomarker. The baseline concentration was established prior to treatment (GD7) and levels were low and similar in all treatment groups. Corticosterone levels were elevated in the perinatal-stress groups compared to the no-stress groups, regardless of Mn exposure, on subsequent time points (GD16, PND9), but were only significantly different on GD16. An analysis of tissue concentrations revealed Mn was elevated similarly in the brain and blood of offspring at PND2 and at PND22 in a significant dose-dependent pattern. Dams also showed a dose-dependent increase in Mn concentrations in the brain and blood; the addition of stress increased the Mn concentrations in the maternal blood but not the brain. Perinatal stress did not alter the effects of Mn on the maternal or offspring somatic endpoints described here.

Thyroid Disruptors: Extrathyroidal Sites of Chemical Action and Neurodevelopmental Outcome-An Examination Using Triclosan and Perfluorohexane Sulfonate.

Many xenobiotics are identified as potential thyroid disruptors due to their action to reduce circulating levels of thyroid hormone, most notably thyroxine (T4). Developmental neurotoxicity is a primary concern for thyroid disrupting chemicals yet correlating the impact of chemically induced changes in serum T4 to perturbed brain development remains elusive. A number of thyroid-specific neurodevelopmental assays have been proposed, based largely on the model thyroid hormone synthesis inhibitor propylthiouracil (PTU). This study examined whether thyroid disrupting chemicals acting distinct from synthesis inhibition would result in the same alterations in brain as expected with PTU. The perfluoroalkyl substance perfluorohexane sulfonate (50 mg/kg/day) and the antimicrobial Triclosan (300 mg/kg/day) were administered to pregnant rats from gestational day 6 to postnatal day (PN) 21, and a number of PTU-defined assays for neurotoxicity evaluated. Both chemicals reduced serum T4 but did not increase thyroid stimulating hormone. Both chemicals increased expression of hepatic metabolism genes, while thyroid hormone-responsive genes in the liver, thyroid gland, and brain were largely unchanged. Brain tissue T4 was reduced in newborns, but despite persistent T4 reductions in serum, had recovered in the PN6 pup brain. Neither treatment resulted in a low dose PTU-like phenotype in either brain morphology or neurobehavior, raising questions for the interpretation of serum biomarkers in regulatory toxicology. They further suggest that reliance on serum hormones as prescriptive of specific neurodevelopmental outcomes may be too simplistic and to understand thyroid-mediated neurotoxicity we must expand our thinking beyond that which follows thyroid hormone synthesis inhibition.

Exposure to Intermittent Noise Exacerbates the Cardiovascular Response of Wistar-Kyoto Rats to Ozone Inhalation and Arrhythmogenic Challenge.

Noise has become a prevalent public health problem across the world. Although there is a significant amount of data demonstrating the harmful effects of noise on the body, very little is known about how it impacts subsequent responses to other environmental stressors like air pollution, which tend to colocalize in urban centers. Therefore, this study was conducted to determine the effect of intermittent noise on cardiovascular function and subsequent responses to ozone (O3). Male Wistar-Kyoto rats implanted with radiotelemeters to non-invasively measure heart rate (HR) and blood pressure (BP), and assess heart rate variability (HRV) and baroreflex sensitivity (BRS) were kept in the quiet or exposed to intermittent white noise (85-90 dB) for one week and then exposed to either O3 (0.8 ppm) or filtered air. Left ventricular function and arrhythmia sensitivity were measured 24 h after exposure. Intermittent noise caused an initial increase in HR and BP, which decreased significantly later in the regimen and coincided with an increase in HRV and BRS. Noise caused HR and BP to be significantly elevated early during O3 and lower at the end when compared to animals kept in the quiet while the increased HRV and BRS persisted during the 24 h after. Lastly, noise increased arrhythmogenesis and may predispose the heart to mechanical function changes after O3. This is the first study to demonstrate that intermittent noise worsens the cardiovascular response to inhaled O3. These effects may occur due to autonomic changes and dysregulation of homeostatic controls, which persist one day after exposure to noise. Hence, co-exposure to noise should be taken into account when assessing the health effects of urban air pollution.

Fetal growth outcomes following peri-implantation exposure of Long-Evans rats to noise and ozone differ by sex.

BACKGROUND: Exposure to air pollution and high levels of noise have both been independently associated with the development of adverse pregnancy outcomes including low birth weight. However, exposure to such environmental stressors rarely occurs in isolation and is often co-localized, especially in large urban areas. METHODS: The purpose of this study was to compare the effects of combined exposure to noise (N) or ozone (O3), compared to either exposure alone. Long-Evans dams were exposed to air or 0.4 ppm ozone for 4 h on gestation day (GD) 5 and 6, coinciding with implantation receptivity. A subset of dams from each exposure group was further exposed to intermittent white noise (~ 85 dB) throughout the dark cycle following each inhalation exposure (n = 14 - 16/group). Uterine artery ultrasound was performed on GD 15 and 21. Fetal growth characteristics and indicators of placental nutrient status were measured at GD 21. RESULTS: Exposure to ozone + quiet (O3 + Q) conditions reduced uterine arterial resistance at GD 15 compared to air + quiet (A + Q) exposure, with no further reduction by GD 21. By contrast, exposure to air + noise (A + N) significantly increased uterine arterial resistance at both GD 15 and 21. Notably, while peri-implantation exposure to O3 + Q conditions reduced male fetal weight at GD 21, this effect was not observed in the air + noise (A + N) or the ozone + noise (O3 + N) exposure groups. Fetal weight in female offspring was not reduced by ozone exposure alone (O3 + Q), nor was it affected by air + noise (A + N) or by combined ozone + noise (O3 + N) exposure. CONCLUSIONS: These data indicate that exposure to ozone and noise differentially impact uterine blood flow, particularly at mid-gestation, with only ozone exposure being associated with sex-dependent fetal growth retardation in male offspring.

Developmental Thyroid Hormone Insufficiency Induces a Cortical Brain Malformation and Learning Impairments: A Cross-Fostering Study.

Thyroid hormones (THs) are essential for brain development, but few rodent models exist that link TH inefficiency to apical neurodevelopmental endpoints. We have previously described a structural anomaly, a heterotopia, in the brains of rats treated in utero with propylthiouracil (PTU). However, how the timing of an exposure relates to this birth defect is unknown. This study seeks to understand how various temporal treatments of the mother relates to TH insufficiency and adverse neurodevelopment of the offspring. Pregnant rats were exposed to PTU (0 or 3 ppm) through the drinking water from gestational day 6 until postnatal day (PN) 14. On PN2 a subset of pups was cross-fostered to a dam of the opposite treatment, to create 4 conditions: pups exposed to PTU prenatally, postnatally, during both periods, or not at all (control). Both PTU and TH concentrations were characterized in the mother and offspring over time, to capture the dynamics of a developmental xenobiotic exposure. Brains of offspring were examined for heterotopia presence and severity, and adult littermates were assessed for memory impairments. Heterotopia were observed under conditions of prenatal exposure, and its severity increased in animals in the most prolonged exposure group. This malformation was also permanent, but not sex biased. In contrast, behavioral impairments were limited to males, and only in animals exposed to PTU during both the gestational and postnatal periods. This suggests a distinct TH-dependent etiology for both phenotypes, and illustrates how timing of hypothyroxinemia can induce abnormal brain structure and function.

Editor's Highlight: Genetic Targets of Acute Toluene Inhalation in Drosophila melanogaster.

Interpretation and use of data from high-throughput assays for chemical toxicity require links between effects at molecular targets and adverse outcomes in whole animals. The well-characterized genome of Drosophila melanogaster provides a potential model system by which phenotypic responses to chemicals can be mapped to genes associated with those responses, which may in turn suggest adverse outcome pathways associated with those genes. To determine the utility of this approach, we used the Drosophila Genetics Reference Panel (DGRP), a collection of ∼200 homozygous lines of fruit flies whose genomes have been sequenced. We quantified toluene-induced suppression of motor activity in 123 lines of these flies during exposure to toluene, a volatile organic compound known to induce narcosis in mammals via its effects on neuronal ion channels. We then applied genome-wide association analyses on this effect of toluene using the DGRP web portal (http://dgrp2.gnets.ncsu.edu), which identified polymorphisms in candidate genes associated with the variation in response to toluene exposure. We tested ∼2 million variants and found 82 polymorphisms located in or near 66 candidate genes that were associated with phenotypic variation for sensitivity to toluene at P < 5 × 10-5, and human orthologs for 52 of these candidate Drosophila genes. None of these orthologs are known to be involved in canonical pathways for mammalian neuronal ion channels, including GABA, glutamate, dopamine, glycine, serotonin, and voltage sensitive calcium channels. Thus this analysis did not reveal a genetic signature consistent with processes previously shown to be involved in toluene-induced narcosis in mammals. The list of the human orthologs included Gene Ontology terms associated with signaling, nervous system development and embryonic morphogenesis; these orthologs may provide insight into potential new pathways that could mediate the narcotic effects of toluene.

Application of Biologically Based Lumping To Investigate the Toxicokinetic Interactions of a Complex Gasoline Mixture.

People are often exposed to complex mixtures of environmental chemicals such as gasoline, tobacco smoke, water contaminants, or food additives. We developed an approach that applies chemical lumping methods to complex mixtures, in this case gasoline, based on biologically relevant parameters used in physiologically based pharmacokinetic (PBPK) modeling. Inhalation exposures were performed with rats to evaluate the performance of our PBPK model and chemical lumping method. There were 109 chemicals identified and quantified in the vapor in the chamber. The time-course toxicokinetic profiles of 10 target chemicals were also determined from blood samples collected during and following the in vivo experiments. A general PBPK model was used to compare the experimental data to the simulated values of blood concentration for 10 target chemicals with various numbers of lumps, iteratively increasing from 0 to 99. Large reductions in simulation error were gained by incorporating enzymatic chemical interactions, in comparison to simulating the individual chemicals separately. The error was further reduced by lumping the 99 nontarget chemicals. The same biologically based lumping approach can be used to simplify any complex mixture with tens, hundreds, or thousands of constituents.

Use of novel inhalation kinetic studies to refine physiologically-based pharmacokinetic models for ethanol in non-pregnant and pregnant rats.

Ethanol (EtOH) exposure induces a variety of concentration-dependent neurological and developmental effects in the rat. Physiologically-based pharmacokinetic (PBPK) models have been used to predict the inhalation exposure concentrations necessary to produce blood EtOH concentrations (BEC) in the range associated with these effects. Previous laboratory reports often lacked sufficient detail to adequately simulate reported exposure scenarios associated with BECs in this range, or lacked data on the time-course of EtOH in target tissues (e.g. brain, liver, eye, fetus). To address these data gaps, inhalation studies were performed at 5000, 10 000, and 21 000 ppm (6 h/d) in non-pregnant female Long-Evans (LE) rats and at 21 000 ppm (6.33 h/d) for 12 d of gestation in pregnant LE rats to evaluate our previously published PBPK models at toxicologically-relevant blood and tissue concentrations. Additionally, nose-only and whole-body plethysmography studies were conducted to refine model descriptions of respiration and uptake within the respiratory tract. The resulting time-course and plethysmography data from these in vivo studies were compared to simulations from our previously published models, after which the models were recalibrated to improve descriptions of tissue dosimetry by accounting for dose-dependencies in pharmacokinetic behavior. Simulations using the recalibrated models reproduced these data from non-pregnant, pregnant, and fetal rats to within a factor of 2 or better across datasets, resulting in a suite of model structures suitable for simulation of a broad range of EtOH exposure scenarios.

Acute inhalation of 2,2,4-trimethylpentane alters visual evoked potentials and signal detection behavior in rats.

The volatile organic compound 2,2,4-trimethylpentane (TMP, "isooctane") is a constituent of gasoline for which the current health effects data are insufficient to permit the US Environmental Protection Agency to conduct a risk assessment. The potential neurological impairment from acute inhalation exposure to TMP was evaluated in adult male Long-Evans rats using both electrophysiological and behavioral assessments. Visual evoked potentials (VEPs) were recorded from rats viewing modulated visual patterns (0.16 cycles per degree visual angle (cpd), 60% contrast, 4.55Hz appear/disappear). Rats (n=7-10/dose) were exposed to TMP vapors in concentrations of 0, 500, or 1000 ppm for 60-min. A VEP was recorded before exposure and at 10 min intervals during exposure and also for 60 min after exposure terminated. The spectral amplitude of the frequency-double component (F2) was significantly reduced after exposure to TMP. In behavioral assessments, rats (n=14) performed an appetitively motivated visual signal detection task while breathing 0, 500, 1500, 1000, 2000, or 2500 ppm TMP for 62 min. Slight reductions in accuracy of performance were observed at the 2500 ppm concentration. Concentrations of TMP in the brain were estimated using a physiologically based pharmacokinetic (PBPK) model to be less than 0.2mM after 62 min at 2500 ppm. Together these data demonstrate that TMP, like other volatile organic substances, impairs neurological function during acute inhalation exposure and that the small magnitude of the observed effects is consistent with the low concentrations of this hydrocarbon that were estimated to reach the CNS.

Acute perchloroethylene exposure alters rat visual-evoked potentials in relation to brain concentrations.

These experiments sought to establish a dose-effect relationship between the concentration of perchloroethylene (PCE) in brain tissue and concurrent changes in visual function. A physiologically based pharmacokinetic (PBPK) model was implemented to predict concentrations of PCE in the brains of adult Long-Evans rats following inhalation exposure. The model was evaluated for performance against tissue concentrations from exposed rats (n = 40) and data from the published scientific literature. Visual function was assessed using steady-state pattern-elicited visual-evoked potentials (VEPs) recorded from rats during exposure to air or PCE in two experiments (total n = 84) with concentrations of PCE ranging from 250 to 4000 ppm. VEP waveforms were submitted to a spectral analysis in which the major response component, F2, occurring at twice the visual stimulation rate, was reduced in amplitude by PCE exposure. The F2 amplitudes were transformed to an effect-magnitude scale ranging from 0 (no effect) to 1 (maximum possible effect), and a logistical function was fit to the transformed values as a function of estimated concurrent brain PCE concentrations. The resultant function described a dose-response relationship between brain PCE concentration and changes in visual function with an ED(10) value of approximately 0.684 mg/l and an ED(50) value of approximately 46.5 mg/l. The results confirmed that visual function was disrupted by acute exposure to PCE, and the PBPK model and logistic model together could be used to make quantitative estimates of the magnitude of deficit to be expected for any given inhalation exposure scenario.

Modeling the toxicokinetics of inhaled toluene in rats: influence of physical activity and feeding status.

Toluene is found in petroleum-based fuels and used as a solvent in consumer products and industrial applications. The critical effects following inhalation exposure involve the brain and nervous system in both humans and experimental animals, whether exposure duration is acute or chronic. The goals of this physiologically based pharmacokinetic (PBPK) model development effort were twofold: (1) to evaluate and explain the influence of feeding status and activity level on toluene pharmacokinetics utilizing our own data from toluene-exposed Long Evans (LE) rats, and (2) to evaluate the ability of the model to simulate data from the published literature and explain differing toluene kinetics. Compartments in the model were lung, slowly and rapidly perfused tissue groups, fat, liver, gut, and brain; tissue transport was blood-flow limited and metabolism occurred in the liver. Chemical-specific parameters and initial organ volumes and blood flow rates were obtained from the literature. Sensitivity analysis revealed that the single most influential parameter for our experimental conditions was alveolar ventilation; other moderately influential parameters (depending upon concentration) included cardiac output, rate of metabolism, and blood flow to fat. Based on both literature review and sensitivity analysis, other parameters (e.g., partition coefficients and metabolic rate parameters) were either well defined (multiple consistent experimental results with low variability) or relatively noninfluential (e.g. organ volumes). Rats that were weight-maintained compared to free-fed rats in our studies could be modeled with a single set of parameters because feeding status did not have a significant impact on toluene pharmacokinetics. Heart rate (HR) measurements in rats performing a lever-pressing task indicated that the HR increased in proportion to task intensity. For rats acclimated to eating in the lab during the day, both sedentary rats and rats performing the lever-pressing task required different alveolar ventilation rates to successfully predict the data. Model evaluation using data from diverse sources together with statistical evaluation of the resulting fits revealed that the model appropriately predicted blood and brain toluene concentrations with some minor exceptions. These results (1) emphasize the importance of experimental conditions and physiological status in explaining differing kinetic data, and (2) demonstrate the need to consider simulation conditions when estimating internal dose metrics for toxicity studies in which kinetic data were not collected.

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.

The role of physical activity and feeding schedule on the kinetics of inhaled and oral toluene in rats.

Published studies of the kinetics of toluene in rats have shown that its concentration in the blood rises during inhalation and falls after exposure stops; a similar uptake profile and longer persistence in blood typify the kinetics after oral exposure. Because rats in these studies are typically inactive during exposure, and behavioral tests of the acute effects of toluene require physical activity and altered feeding schedules, this study examined the role of physical activity and feeding status on the uptake of toluene given by the two routes. Two groups of adult male Long-Evans rats were conditioned to eat in the lab during the day. A group of "conditioned-active" (C-A) rats performed a lever-pressing task (LPT) for 1 h, either while inhaling toluene vapor (2000 ppm) or after a gavage dose (800 mg/kg toluene in corn oil). Another group of "conditioned-sedentary" (C-S) rats was dosed similarly but did not perform the LPT. A third group of "home cage" (HC) rats was not conditioned to eat during the day, but was maintained under typical laboratory conditions (eating at night in the home cage) before receiving toluene by gavage. In the conditioned rats, physical activity during inhalation exposure increased the concentrations of toluene in blood (from 35.8 +/- 2.5 to 45.2 +/- 3.2 mg/L after 60 min) and brain (from 73.4 +/- 5.3 to 103.0 +/- 3.8 mg/L after 60 min), but did not affect those concentrations after oral toluene. The time course of the uptake of toluene into blood and brain of HC rats followed that of published data. In contrast, toluene concentrations in the blood and brain of orally dosed conditioned rats fell rapidly compared to HC rats and published data (at 60 min after dosing, blood concentrations were: C-S rats, 17.2 +/- 1.7 mg/L; HC rats, 69.4 +/- 9.6 mg/L; and brain concentrations were: C-S rats, 30.9 +/- 5.0 mg/L; HC rats, 96.6 +/- 18.5 mg/L). These studies demonstrate the importance of physical activity for the uptake of inhaled toluene, and the importance of feeding conditions for the elimination of oral toluene.

A dosimetric analysis of the acute behavioral effects of inhaled toluene in rats.

Knowledge of the appropriate metric of dose for a toxic chemical facilitates quantitative extrapolation of toxicity observed in the laboratory to the risk of adverse effects in the human population. Here, we utilize a physiologically based toxicokinetic (PBTK) model for toluene, a common volatile organic compound (VOC), to illustrate that its acute behavioral effects in rats can be quantitatively predicted on the basis of its concentration in the brain. Rats previously trained to perform a visual signal detection task for food reward performed the task while inhaling toluene (0, 1200, 1600, 2000, and 2400 ppm in different test sessions). Accuracy and speed of responding were both decreased by toluene; the magnitude of these effects increased with increasing concentration of the vapor and with increasing duration of exposure. Converting the exposure conditions to brain toluene concentration using the PBTK model yielded a family of overlapping curves for each end point, illustrating that the effects of toluene can be described quantitatively by its internal dose at the time of behavioral assessment. No other dose metric, including inhaled toluene concentration, duration of exposure, the area under the curve of either exposure (ppm h), or modeled brain toluene concentration (mg-h/kg), provided unambiguous predictions of effect. Thus, the acute behavioral effects of toluene (and of other VOCs with a similar mode of action) can be predicted for complex exposure scenarios by simulations that estimate the concentration of the VOC in the brain from the exposure scenario.