PBPK modeling of impact of nonalcoholic fatty liver disease on toxicokinetics of perchloroethylene in mice.

BACKGROUND: Nonalcoholic fatty liver disease (NAFLD), a major cause of chronic liver disease in the Western countries with increasing prevalence worldwide, may substantially affect chemical toxicokinetics and thereby modulate chemical toxicity. OBJECTIVES: This study aims to use physiologically-based pharmacokinetic (PBPK) modeling to characterize the impact of NAFLD on toxicokinetics of perchloroethylene (perc). METHODS: Quantitative measures of physiological and biochemical changes associated with the presence of NAFLD induced by high-fat or methionine/choline-deficient diets in C57B1/6 J mice are incorporated into a previously developed PBPK model for perc and its oxidative and conjugative metabolites. Impacts on liver fat and volume, as well as blood:air and liver:air partition coefficients, are incorporated into the model. Hierarchical Bayesian population analysis using Markov chain Monte Carlo simulation is conducted to characterize uncertainty, as well as disease-induced variability in toxicokinetics. RESULTS: NAFLD has a major effect on toxicokinetics of perc, with greater oxidative and lower conjugative metabolism as compared to healthy mice. The NAFLD-updated PBPK model accurately predicts in vivo metabolism of perc through oxidative and conjugative pathways in all tissues across disease states and strains, but underestimated parent compound concentrations in blood and liver of NAFLD mice. CONCLUSIONS: We demonstrate the application of PBPK modeling to predict the effects of pre-existing disease conditions as a variability factor in perc metabolism. These results suggest that non-genetic factors such as diet and pre-existing disease can be as influential as genetic factors in altering toxicokinetics of perc, and thus are likely contribute substantially to population variation in its adverse effects.

Comparative analysis of metabolism of trichloroethylene and tetrachloroethylene among mouse tissues and strains.

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chemicals that are metabolized through oxidation and glutathione conjugation pathways. Both chemicals have been shown to elicit liver and kidney toxicity in rodents and humans; however, TCE has been studied much more extensively in terms of both metabolism and toxicity. Despite their qualitative similarities, quantitative comparison of tissue- and strain-specific metabolism of TCE and PCE has not been performed. To fill this gap, we conducted a comparative toxicokinetic study where equimolar single oral doses of TCE (800 mg/kg) or PCE (1000 mg/kg) were administered to male mice of C57BL/6J, B6C3F1/J, and NZW/LacJ strains. Samples of liver, kidney, serum, brain, and lung were obtained for up to 36 h after dosing. For each tissue, concentrations of parent compounds, as well as their oxidative and glutathione conjugation metabolites were measured and concentration-time profiles constructed. A multi-compartment toxicokinetic model was developed to quantitatively compare TCE and PCE metabolism. As expected, the flux through oxidation metabolism pathway predominated over that through conjugation across all mouse strains examined, it is 1,200-3,800 fold higher for TCE and 26-34 fold higher for PCE. However, the flux through glutathione conjugation, albeit a minor metabolic pathway, was 21-fold higher for PCE as compared to TCE. The degree of inter-strain variability was greatest for oxidative metabolites in TCE-treated and for glutathione conjugation metabolites in PCE-treated mice. This study provides critical data for quantitative comparisons of TCE and PCE metabolism, and may explain the differences in organ-specific toxicity between these structurally similar chemicals.

Simultaneous detection of the tetrachloroethylene metabolites S-(1,2,2-trichlorovinyl) glutathione, S-(1,2,2-trichlorovinyl)-L-cysteine, and N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine in multiple mouse tissues via ultra-high performance liquid chromatography electrospray ionization tandem mass spectrometry.

Tetrachloroethylene (perchloroethylene; PERC) is a high-production volume chemical and ubiquitous environmental contaminant that is hazardous to human health. Toxicity attributed to PERC is mediated through oxidative and glutathione (GSH) conjugation metabolites. The conjugation of PERC by glutathione-s-transferase to generate S-(1,2,2-trichlorovinyl) glutathione (TCVG), which is subsequently metabolized to form S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) is of special importance to human health. Specifically, TCVC may be metabolized to N-acetyl-S-(1,2,2-trichlorovinyl)-L-cysteine (NAcTCVC) which is excreted through urine, or to electrophilic metabolites that are nephrotoxic and mutagenic. Little is known regarding toxicokinetics of TCVG, TCVC, and NAcTCVC as analytical methods for simultaneous determination of these metabolites in tissues have not yet been reported. Hence, an ultra-high-performance liquid chromatography electrospray ionization tandem mass spectrometry-based method was developed for analysis of TCVG, TCVC, and NAcTCVC in liver, kidneys, serum, and urine. The method is rapid, sensitive, robust, and selective for detection all three analytes in every tissue examined, with limits of detection (LOD) ranging from 1.8 to 68.2 femtomoles on column, depending on the analyte and tissue matrix. This method was applied to quantify levels of TCVG, TCVC, and NAcTCVC in tissues from mice treated with PERC (10 to 1000 mg/kg, orally) with limits of quantitation (LOQ) of 1-2.5 pmol/g in liver, 1-10 pmol/g in kidney, 1-2.5 pmol/ml in serum, and 2.5-5 pmol/ml in urine. This method is useful for further characterization of the GSH conjugative pathway of PERC in vivo and improved understanding of PERC toxicity.

Assessment of Exposure to Perchloroethylene and its Clinical Repercussions for 50 Dry-Cleaning Employees.

The purpose of this article is the assessment of occupational exposure to perchloroethylene (PCE) and its clinical repercussions for dry-cleaning employees. The authors measured atmospheric levels of PCE and blood levels in a population of 50 exposed employees then conducted a study of clinical symptomatology in exposed and non-exposed subjects linked with this solvent. Fifty employees and 95 controls were studied. The median value of atmospheric PCE was 7 ppm (0.22-33), and the median blood level of PCE was 73.6 µg/l (11.8-144). These levels were correlated statistically to the action of sludge scraping and to the existence of automatic scrapers (p < 0.01). Eight percent of PCE blood levels were higher than the biological levels recently set in France. The exposed population did not show excessive signs of drowsiness nor of pre-narcotic syndrome or other symptoms studied. Dry-cleaning employees were exposed to PCE at atmospheric levels lower than the French and American chronic recommended exposure levels but some results were higher than recommended values. For PCE blood levels for the general working population, results were respectively lower than French and American national recommended levels in 92% and 94% cases. Risk should be considered, however, carefully in women of childbearing age, as 64% exceeded the recommended blood levels for pregnant women. This exposure did not generate any studied neurobehavioral symptomatology.

Biomonitoring study of dry cleaning workers using cytogenetic tests and the comet assay.

Perchloroethylene (PCE) is the main solvent used in the dry cleaning industry worldwide. The aim of the present work was to evaluate the genotoxic potential of occupational exposure to PCE in dry cleaning workers. The study was carried out in 59 volunteers (30 workers, 29 controls). The genotoxic effect was evaluated by analyzing chromosome aberrations (CAs), and micronuclei (MN) and DNA damage (assessed by the comet assay) in peripheral blood lymphocytes. Environmental monitoring of exposure was carried out on personal breathing zone air samples collected during two consecutive working days by measuring the concentration of PCE air levels. The mean PCE concentration in workplace air of dry cleaning workers was 31.40 mg/m(3). There were no significant differences in CA frequency between dry cleaning workers and the controls, but analysis showed a significant association of CA frequency with employment duration and frequency of exposure to PCE. The MN frequency and DNA damage detected by alkaline comet assay were significantly increased in dry cleaning workers compared to the controls. The results suggest that (a) chronic occupational exposure to dry cleaning solvents below permissible occupational exposure limit of 70 mg/m(3) (i.e., ~10.3 ppm) may lead to an increased risk of genetic damage among dry cleaning workers, and (b) CA, MN tests, and comet assay are useful to monitor populations exposed to low doses of PCE.

Socioeconomic disparities in indoor air, breath, and blood perchloroethylene level among adult and child residents of buildings with or without a dry cleaner.

In many cities, dry cleaners using perchloroethylene are frequently located in multifamily residential buildings and often cause elevated indoor air levels of perchloroethylene throughout the building. To assess individual perchloroethylene exposures associated with co-located dry cleaners, we measured perchloroethylene in residential indoor air, and in blood and breath of adults and children residing in buildings with a dry cleaner as part of the New York City (NYC) Perc Project. We also measured perchloroethylene in indoor air, and in blood and breath of residents of buildings without a dry cleaner for comparison. Here, we evaluate whether an environmental disparity in perchloroethylene exposures is present. Study participants are stratified by residential building type (dry cleaner or reference) and socioeconomic characteristics (race/ethnicity and income); measures of perchloroethylene exposure are examined; and, the influence of stratified variables and other factors on perchloroethylene exposure is assessed using multivariate regression. All measures of perchloroethylene exposure for residents of buildings with a dry cleaner indicated a socioeconomic disparity. Mean indoor air perchloroethylene levels were about five times higher in minority (82.5 ug/m(3)) than in non-minority (16.5 ug/m(3)) households, and about six times higher in low-income (105.5 ug/m(3)) than in high income (17.8 ug/m(3)) households. Mean blood perchloroethylene levels in minority children (0.27 ng/mL) and adults (0.46 ng/mL) were about two and three times higher than in non-minority children (0.12 ng/mL) and adults (0.15 ng/mL), respectively. Mean blood perchloroethylene levels in low income children (0.34 ng/mL) and adults (0.62 ng/mL) were about three and four times higher than in high income children (0.11 ng/mL) and adults (0.14 ng/mL), respectively. A less marked socioeconomic disparity was observed in perchloroethylene breath levels with minority and low income residents having slightly higher levels than non-minority and high income residents. Multivariate regression affirmed that indoor air perchloroethylene level in dry cleaner buildings was the single most important factor determining perchloroethylene in blood and breath. Neither age, gender, nor socioeconomic status significantly influenced perchloroethylene levels in breath or blood. We previously reported that increased indoor air, breath, and blood perchloroethylene levels among NYC Perc Project child participants were associated with an increased risk for slightly altered vision. Thus, the disproportionately elevated perchloroethylene exposures of minority and low-income child residents of buildings with a dry cleaner shown here constitutes an environmental exposure disparity with potential public health consequences. Among residents of buildings without a dry cleaner, we observed some small increases in perchloroethylene breath and blood levels among non-minority or high income residents compared to minority or low income residents. These differences were not attributable to differences in indoor air levels of perchloroethylene which did not differ across socioeconomic categories, but appear to be associated with more frequent exposures dry cleaned garments.

Biological monitoring of exposure to perchloroethylene in dry cleaning workers.

BACKGROUND: Perchloroethylene (PCE) is the most widely used solvent in dry cleaning. OBJECTIVES: The aim was to evaluate PCE pollution and to identify the most reliable biological indicators for the assessment of workers' exposure. METHODS: The study was performed in 40 dry cleaning shops covering a total of 71 subjects. Environmental monitoring was carried out with personal diffusive samplers (Radiello) for the entire work shift; biological monitoring was performed by measuring PCE in urine and blood and trichloroacetic acid (TCA) in urine on Thursday evening at end-of shift and on Friday morning pre-shift. RESULTS: The mean concentration of PCE in air was 52.32 mg/m3, about 30% of the TLV-TWA and the mean value of the PCE inpre-shift blood samples was 0.304 mg/l, slightly more than 50% of the BEI. In dry cleaning shops employing less than 3 persons PCE in air exceeded the TLV-TWA in 7.8% of cases; the size of the shops was inversely related to pollution. Statistically significant correlations were found between PCE exposure and PCE in blood end-of-shift (r = 0.67) and pre-shift (r = 0.70), and PCE in urine end-of-shift (r = 0.68); no correlation was found between exposure and PCE in urine pre-shift and urinary TCA. CONCLUSIONS: Dry cleaning shops still register conditions of exposure and pollution by PCE, although to a lesser extent than in the past. The most reliable indicators for biological monitoring are CE in end-of-shift urine and PCE in blood both at end-of-shift and pre-shift at the end of the workweek.

Visual contrast sensitivity in children exposed to tetrachloroethylene.

This study examined relationships between indoor air, breath, and blood tetrachloroethylene (perc) levels and visual contrast sensitivity (VCS) among adult and child residents of buildings with or without a colocated dry cleaner using perc. Decreasing trends in proportions of adults or children with maximum VCS scores indicated decreased VCS at a single spatial frequency (12 cycles per degree [cpd]) among children residing in buildings with colocated dry cleaners when indoor air perc level averaged 336 µg/m³; breath perc level averaged 159.5 µg/m³; and blood perc level averaged 0.51 µg/L. Adjusted logistic regression indicated that increases in indoor air, breath, and blood perc levels among all child participants significantly increased the odds for decreased VCS at 12 cpd. Adult VCS was not significantly decreased by increasing indoor air, breath, or blood perc level. These results suggest that elevated residential perc exposures may alter children's VCS, a possible subclinical central nervous system effect.

Immunotoxicity and hematotoxicity induced by tetrachloroethylene in egyptian dry cleaning workers.

The immune and hematological systems can be a target for environmental contaminants with potential adverse effects, so the purpose of this study is to provide documentation on immunotoxicity and hematotoxicity of tetrachloroethylene, which is widely used in dry cleaning in Egypt. This study was carried out on 80 adult males. Subjects designated as controls (n = 40) were healthy persons and others were tetrachloroethylene-exposed dry-cleaning workers (n = 40). The controls and tetrachloroethylene-exposed workers were then divided into four equal groups (20 individuals/group): group I, control group never smoking; group II, smoking control group; and groups III and IV, tetrachloroethylene-exposed nonsmoking and smoking workers, respectively. Blood level of tetrachloroethylene, complete blood count, immunoglobulins (IgA, IgM, IgG, and IgE), the total numbers of white blood cells (WBC), and leukocyte differential counts, as well as interferon gamma (IFN-gamma) and interleukin-4 (IL-4), were measured. The immunotoxicity of tetrachloroethylene appeared in the form of an increase in serum immunoglobulin E in nonsmoking and smoking tetrachloroethylene-exposed workers, while the serum immunoglobulins A, M, and G levels showed no significant change in all studied groups. In addition, our results demonstrated a significant increase in white cell count, lymphocytes, natural killer (NK; CD3+CD16CD56+) cells, and B (CD19+) lymphocytes. The increase in WBC and lymphocytes may be attributed to allergic reaction. Moreover, serum and lymphocytic interlukin-4 levels were significantly increased in nonsmoking and smoking tetrachloroethylene-exposed workers. Tetrachloroethylene exposure is associated with immunotoxicity, which may lead to the augmentation of allergic diseases or appearance of autoimmune reaction.

Bayesian analysis of a physiologically based pharmacokinetic model for perchloroethylene in humans.

Perchloroethylene (PCE) is a widely distributed pollutant in the environment, and is the primary chemical used in dry cleaning. PCE-induced liver cancer was observed in mice, and central nervous system (CNS) effects were reported in dry-cleaning workers. To support reconstruction of human PCE exposures, including the potential for CNS effects, an existing physiologically based pharmacokinetic (PBPK) model for PCE in the human (Covington et al., 2007) was modified by adding a brain compartment. A Bayesian approach, using Markov chain Monte Carlo (MCMC) analysis, was employed to re-estimate the parameters in the modified model by combining information from prior distributions for the model parameters and experimental data. Experimental data were obtained from five different human pharmacokinetic studies of PCE inhalation exposures ranging from 150 ppm to as low as 0.495 ppm. The data include alveolar or exhaled breath concentrations of PCE, blood concentrations of PCE and trichloroacetic acid (TCA), and urinary excretion of TCA. The PBPK model was used to predict target tissue dosimetry of PCE and its key metabolite, TCA, during and after the inhalation exposures. Posterior analysis was performed to see whether convergence criteria for each parameter were satisfied and whether the model with posterior distributions may be used to make accurate predictions of human kinetic data. With posteriors, the trend of percent of PCE metabolized in the liver at low concentrations was predicted under different exposure conditions. The 95th percentile for the fraction PCE metabolized at a concentration of 1 ppb was estimated to be 1.89%.

Biological exposure assessment to tetrachloroethylene for workers in the dry cleaning industry.

BACKGROUND: The purpose of this study was to assess the feasibility of conducting biological tetrachloroethylene (perchloroethylene, PCE) exposure assessments of dry cleaning employees in conjunction with evaluation of possible PCE health effects. METHODS: Eighteen women from four dry cleaning facilities in southwestern Ohio were monitored in a pilot study of workers with PCE exposure. Personal breathing zone samples were collected from each employee on two consecutive work days. Biological monitoring included a single measurement of PCE in blood and multiple measurements of pre- and post-shift PCE in exhaled breath and trichloroacetic acid (TCA) in urine. RESULTS: Post-shift PCE in exhaled breath gradually increased throughout the work week. Statistically significant correlations were observed among the exposure indices. Decreases in PCE in exhaled breath and TCA in urine were observed after two days without exposure to PCE. A mixed-effects model identified statistically significant associations between PCE in exhaled breath and airborne PCE time weighted average (TWA) after adjusting for a random participant effect and fixed effects of time and body mass index. CONCLUSION: Although comprehensive, our sampling strategy was challenging to implement due to fluctuating work schedules and the number (pre- and post-shift on three consecutive days) and multiplicity (air, blood, exhaled breath, and urine) of samples collected. PCE in blood is the preferred biological index to monitor exposures, but may make recruitment difficult. PCE TWA sampling is an appropriate surrogate, although more field intensive. Repeated measures of exposure and mixed-effects modeling may be required for future studies due to high within-subject variability. Workers should be monitored over a long enough period of time to allow the use of a lag term.

Toxicokinetics of inhaled trichloroethylene and tetrachloroethylene in humans at 1 ppm: empirical results and comparisons with previous studies.

Trichloroethylene (TRI) and tetrachloroethylene (TETRA) are solvents that have been widely used in a variety of industries, and both are widespread environmental contaminants. In order to provide a better basis for understanding their toxicokinetics at environmental exposures, seven human volunteers were exposed by inhalation to 1 ppm of TRI or TETRA for 6 h, with biological samples collected for analysis during exposure and up to 6-days postexposure. Concentrations of TRI, TETRA, free trichloroethanol (TCOH), total TCOH (free TCOH plus glucuronidated TCOH), and trichloroacetic acid (TCA) were determined in blood and urine; TRI and TETRA concentrations were measured in alveolar breath. Toxicokinetic time courses and empirical analyses of classical toxicokinetic parameters were compared with those reported in previous human volunteer studies, most of which involved exposures that were at least 10-fold higher. Qualitatively, TRI and TETRA toxicokinetics were consistent with previous human studies. Quantitatively, alveolar retention and clearance by exhalation were similar to those found previously but blood and urine data suggest a number of possible toxicokinetic differences. For TRI, data from the current study support lower apparent blood-air partition coefficients, greater apparent metabolic clearance, less TCA production, and greater glucuronidation of TCOH as compared to previous studies. For TETRA, the current data suggest TCA formation that is similar or slightly lower than that of previous studies. Variability and uncertainty in empirical estimates of total TETRA metabolism are substantial, with confidence intervals among different studies substantially overlapping. Relative contributions to observed differences from concentration-dependent toxicokinetics and interindividual and interoccasion variability remain to be determined.

Perchloroethylene in alveolar air, blood, and urine as biologic indices of low-level exposure.

We studied the reliability of biologic indices for monitoring perchlorethylene (PCE) exposure at low environmental solvent concentrations. Environmental monitoring was performed by personal sampling, biologic monitoring by measuring PCE in alveolar air (PCE-Alv), blood (PCE-B), and urine (PCE-U) in 26 low-exposed dry-cleaners. Correlation coefficients (r) between environmental PCE and PCE-B, PCE-Alv, and PCE-U were 0.94, 0.81, and 0.67 respectively. A high correlation was also found among biologic indices: r value was 0.96 between PCE-B and PCE-Alv, 0.95 between PCE-B and PCE-U, and 0.87 between alveolar PCE-Alv and PCE-U. The examined biologic indices proved sensitive enough for biologic monitoring of low exposure to PCE and can give substantially similar information in terms of exposure evaluation. PCE-Alv offers some advantages because it correlated better with exposure and is analytically simpler.

Tetrachloroethylene and trichloroethylene fatality: case report and simple headspace SPME-capillary gas chromatographic determination in tissues.

We describe a simple, precise, and sensitive assay of tetrachloroethylene and trichloroethylene in tissues, suitable both for emergency cases and forensic medicine. The method employs headspace solid phase microextraction-capillary gas chromatography and electron capture detection. The case is relative to a 45-year-old woman discovered unconscious in a laundry area. The concentrations of the solvents in tissues were determined and compared to other previously published fatalities.

[Determination of volatile organic compounds in blood by headspace solid-phase microextraction-gas chromatography].

The headspace solid-phase microextraction (HS-SPME) is a novel extraction technique and has been developed rapidly. It is a fast, simple, solventless and sensitive method for sampling, separating, extracting, injecting and analyzing volatile organic compounds. This paper presents the research work in detecting volatile organic compounds(including ten compounds) in blood. The extraction fiber is made by fused-silica fiber with 100 microns polydimethylsiloxane (PDMS). The extraction time of the method was 10 min. The thermal desorption time was 1 min. It was found that the optimized location of the extraction fiber in the injector of GC was to put the whole needle in the injector. The precision of the method was determined to be less 5% relative standard deviation (RSD). The linear range of the detection was rather wide. The lowest detectin limits (LODs) were all < or = 5 ng/ml.

Optic neuritis with residual tunnel vision in perchloroethylene toxicity.

CASE REPORT: In a 57-year-old female owner of a dry-cleaning shop, we describe the association of severe bilateral optic neuritis with unexpectedly high concentrations of perchloroethylene/metabolites in the blood and of chloroform in urine. Visual disturbances consisted of complete blindness for 9 days in the left eye, for 11 days in the right eye, with bright phosphenes and pain on eye rotation. Only central (2-3 degrees radius) vision recovered in the following months. CONCLUSION: Although environmental concentrations of perchloroethylene were within normal limits, we measured five-fold increases in vapors emitted when ironing freshly dry-cleaned fabrics, and suggest that inhalation of perchloroethylene vapors was the cause of this case of ocular nerve toxicity, recapitulating a previous report of major perchloroethylene toxicity.

Tetrachloroethylene intoxication in an autoerotic fatality.

This case report describes an accidental death due to the inhalation of tetrachloroethylene during an autoerotic episode. Tetrachloroethylene was administered from a can of Fix-A-Flat tire repair. Analysis of tetrachloroethylene was performed using headspace gas chromatography and electron capture detection. The blood tetrachloroethylene concentration of 62 mg/L was consistent with acute tetrachloroethylene intoxication.

Schedule-controlled operant behavior of rats following oral administration of perchloroethylene: time course and relationship to blood and brain solvent levels.

Previous studies have indicated that human exposure to perchloroethylene (PCE) produces subtle behavioral changes and other neurological effects at concentration at or below the current occupational exposure limit. Since comparable effects in animals may be reflected by changes in schedule-controlled operant behavior, the ability of orally administered PCE to alter fixed-ratio (FR) responding for a food reward was investigated in male Sprague-Dawley rats. Furthermore, since behavioral effects of solvents are likely to be more closely related to blood or target tissue (i.e, brain) concentrations than administered dose, the relationship between the pharmacokinetic distribution of PCE and its effects on operant responding was also evaluated. Rats trained to lever-press for evaporated milk on an FR-40 reinforcement schedule were gavaged with 160 or 480 mg/kg PCE and immediately placed in an operant test cage for 90 min. Separate animals gavaged with equivalent doses of PCE were used to determine profiles of blood and brain concentrations versus time. Perchloroethylene produced changes in responding that varied not only with dose but also among animals receiving the same dose. Changes in the response rates of rats receiving 160 mg/kg PCE were either not readily apparent, restricted to the first 5 min of the operant session, or attributable to gavage stress and the dosing vehicle. However, 480 mg/kg produced either an immediate suppression of responding for 15-30 min before a rapid recovery to control rates or a complete elimination of lever-pressing for the majority of the operant session. Although the two doses of PCE produced markedly different effects on operant behavior during the first 30 min of exposure, differences in brain concentrations of PCE were minimal. Furthermore, the majority of animals receiving 480 mg/kg PCE fully recovered from response suppression while blood and brain levels of the solvent continued to rise. Thus, relationships between blood and brain PCE levels and performance impairment were not discernible over the monitored time course. Since the rapid onset of response suppression suggests that the precipitating event occurs within the first few minutes of exposure, it is possible that altered responding is related to the rate of increase in blood or brain concentrations rather than the absolute solvent concentrations themselves. The relationship between the pharmacokinetic distribution of solvents and their effects on the central nervous system is obviously complex and may involve acute neuronal adaptation as well as the dynamics of solvent distribution among the various body compartments.