Persistent effects of subchronic toluene exposure on spatial learning and memory, dopamine-mediated locomotor activity and dopamine D2 agonist binding in the rat

Effects of housing condition on the development and persistence of addictive-like behavior induced by toluene

Environmental factors can modify addictive responses induced by drugs of abuse; however, little is known about the impact of environmental conditions on behavioral responses induced by inhalants. In this study, we analyzed the effects of housing conditions, considering environmental enrichment (EE; n = 10), social isolation (SI; n = 10), and standard housing (STD; n = 10), as positive, negative, and control environments, respectively, on the development and persistence of behavioral sensitization induced by toluene. Mice exposed to air were used as a comparative control groups for each housing condition (EE: n = 11, SI: n = 10 and STD: n = 11). Results showed that a history of toluene exposure induced the development of locomotor sensitization in mice, independent of their housing conditions. However, SI increased the expression of behavioral sensitization to toluene after a drug-free period.

Recommedation for an occupational exposure limit for toluene

The Lower Olefins and Aromatics (LOA) REACH Consortium, which includes toluene registrants in the EU, established a Working Group (WG) to conduct a review of the occupational exposure limit (OEL) for toluene. The review focussed on CNS and neuro-behavioural toxicity, ototoxicity, effects on colour vision, reproductive and developmental effects, as safety signals for these effects were identified. The WG also examined the need for a skin notation and/or a short-term exposure limit (STEL). The WG critically reviewed and discussed the strengths and weaknesses of the available published information describing the effects of toluene in animals and humans, to assess its adequacy as a potential point of departure for the establishment of an OEL for toluene and to derive an OEL. As a result, the WG recommendation for a toluene OEL is 20 ppm 8 h TWA, with a 15 min STEL of 100 ppm and a skin notation.

Effects of inhaled combined Benzene, Toluene, Ethylbenzene, and Xylenes (BTEX): Toward an environmental exposure model

Combined environmental exposures to the volatile organic compounds (VOCs) Benzene, Toluene, Ethylbenzene, and Xylene (BTEX) pose clear risks to public health. Research into these risks is under-studied even as BTEX levels in the atmosphere are predicted to rise. This review focuses on the available literature using single- and combined-BTEX component inhaled solvent exposures in animal models, necessarily also drawing on findings from models of inhalant abuse and occupational exposures. Health effects of these exposures are discussed for multiple organ systems, but with particular attention on neurobehavioral outcomes such as locomotor activity, impulsivity, learning, and psychopharmacological responses. It is clear that animal models have significant differences in the concentrations, durations and patterns of exposure. Experimental evidence of the deleterious health and neurobehavioral consequences of exposures to the individual components of BTEX were found, but these effects were typically assessed using concentrations and exposure patterns not characteristic of environmental exposure. Future studies with animal models designed appropriately to explore combined BTEX will be necessary and advantageous to discovering health outcomes and more subtle neurobehavioral impacts of long-term environmental exposures.

Prenatal Exposure to Paint Thinner Alters Postnatal Development and Behavior in Mice

Occupational exposure and sniffing of volatile organic solvents continue to be a worldwide health problem, raising the risk for teratogenic sequelae of maternal inhalant abuse. Real life exposures usually involve simultaneous exposures to multiple solvents, and almost all the abused solvents contain a mixture of two or more different volatile compounds. However, several studies examined the teratogenicity due to industrial exposure to a single volatile solvent but investigating the teratogenic potential of complex chemical mixture such as thinner remains unexplored. This study was undertaken to evaluate developmental neurotoxicity of paint thinner using a mouse model. Mated female mice (N = 21) were, therefore, exposed to repeated and brief inhalation episodes of 0, 300 or 600 ppm of thinner during the entire period of pregnancy. Females weigh was recorded and their standard fertility and reproductive parameters were assessed. After birth postnatal day 1 (PND1), offspring (N = 88) length and body weight were measured in a daily basis. At PND5, the pups were assessed for their postnatal growth, physical maturation, reflex development, neuromotor abilities, sensory function, activity level, anxiety, depression, learning and memory functions. At adulthood, structural changes of the hippocampus were examined by estimating the total volume of the dentate gyrus. Except one case of thinner induced abortion at the higher dose, our results showed that the prenatal exposure to the solvent did not cause any maternal toxicity or decrease in the viability of the offspring. Therefore, a lower birth weight, decrease in the litter size and delayed reflexes ontogeny were registered in prenatally exposed offspring to both 300 ppm and 600 ppm of thinner. In addition, prenatally exposure to thinner resulted in increased anxiolytic- and depression-like behaviors. In contrast, impaired learning and memory functions and decreased hippocampal dentate gyrus volume were revealed only in the prenatally treated offspring by 600 ppm of thinner. Based on these results, we can conclude that prenatally exposure to paint thinner causes a long-lasting developmental neurotoxicity and alters a wide range of behavioral functions in mice. This shows the risk that mothers who abuse thinner paint expose their offspring.

Acute and subchronic toxicity of inhaled toluene in male Long-Evans rats: Oxidative stress markers in brain

The effects of exposure to volatile organic compounds (VOCs), which are of concern to the EPA, are poorly understood, in part because of insufficient characterization of how human exposure duration impacts VOC effects. Two inhalation studies with multiple endpoints, one acute and one subchronic, were conducted to seek effects of the VOC, toluene, in rats and to compare the effects between acute and subchronic exposures. Adult male Long-Evans rats were exposed to toluene vapor (n=6 per group) at a concentration of 0 or 1019 ± 14 ppm for 6h in the acute study and at 0 ± 0, 10 ± 1.4, 97 ± 7, or 995 ± 43 ppm for 6h/d, 5d/week for 13 weeks in the subchronic study. For the acute study, brains were dissected on ice within 30 min of the end of exposure, while for the subchronic study, brains were dissected 18 h after the last exposure. Frontal cortex, hippocampus, cerebellum, and striatum were assayed for a variety of oxidative stress (OS) parameters including total aconitase (TA), protein carbonyls, glutathione peroxidase (GPX), glutathione reductase (GRD), glutathione transferase (GST), γ-glutamylcysteine synthetase (GCS), superoxide dismutase (SOD), total antioxidants (TAS), NADPH quinone oxidoreductase-1 (NQO1), and NADH ubiquinone reductase (UBIQ-RD) activities using commercially available kits. Following acute exposure, UBIQ-RD, GCS and GRD were increased significantly only in the cerebellum, while TAS was increased in frontal cortex. On the other hand, subchronic exposure affected several OS markers including increases in NQO1 and UBIQ-RD. The effect of subchronic toluene exposure on SOD and TAS was greater in the striatum than in the other brain regions. TA activity (involved in maintaining iron homeostasis and an indicator of DNA damage) was inhibited in striatum and cerebellum, increased in hippocampus, and unchanged in frontal cortex. Protein carbonyls increased significantly in both the frontal cortex and cerebellum. In general, the results showed that acute exposure to toluene affected OS parameters to a lesser extent than did subchronic exposure. These results suggest that toluene exposure induces OS in the brain and this may be a component of an adverse outcome pathway for some of the neurotoxic effects reported following toluene exposure.

Neurotoxicity following acute inhalation of aerosols generated during resistance spot weld-bonding of carbon steel

Welding generates complex metal aerosols, inhalation of which is linked to adverse health effects among welders. An important health concern of welding fume (WF) exposure is neurological dysfunction akin to Parkinson's disease (PD). Some applications in manufacturing industry employ a variant welding technology known as "weld-bonding" that utilizes resistance spot welding, in combination with adhesives, for metal-to-metal welding. The presence of adhesives raises additional concerns about worker exposure to potentially toxic components like Methyl Methacrylate, Bisphenol A and volatile organic compounds (VOCs). Here, we investigated the potential neurotoxicological effects of exposure to welding aerosols generated during weld-bonding. Male Sprague-Dawley rats were exposed (25 mg/m³ targeted concentration; 4 h/day × 13 days) by whole-body inhalation to filtered air or aerosols generated by either weld-bonding with sparking (high metal, low VOCs; HM) or without sparking (low metal; high VOCs; LM). Fumes generated under these conditions exhibited complex aerosols that contained both metal oxide particulates and VOCs. LM aerosols contained a greater fraction of VOCs than HM, which comprised largely metal particulates of ultrafine morphology. Short-term exposure to LM aerosols caused distinct changes in the levels of the neurotransmitters, dopamine (DA) and serotonin (5-HT), in various brain areas examined. LM aerosols also specifically decreased the mRNA expression of the olfactory marker protein (Omp) and tyrosine hydroxylase (Th) in the olfactory bulb. Consistent with the decrease in Th, LM also reduced the expression of dopamine transporter (Slc6a3; Dat), as well as, dopamine D2 receptor (Drd2) in the olfactory bulb. In contrast, HM aerosols induced the expression of Th and dopamine D5 receptor (Drd5) mRNAs, elicited neuroinflammation and blood-brain barrier-related changes in the olfactory bulb, but did not alter the expression of Omp. Our findings divulge the differential effects of LM and HM aerosols in the brain and suggest that exposure to weld-bonding aerosols can potentially elicit neurotoxicity following a short-term exposure. However, further investigations are warranted to determine if the aerosols generated by weld-bonding can contribute to persistent long-term neurological deficits and/or neurodegeneration.

Specific impairments in instrumental learning following chronic intermittent toluene inhalation in adolescent rats

RATIONALE

Inhalant abuse is prevalent in adolescent populations, with chronic use resulting in neurobiological and cognitive abnormalities in adulthood. However, the nature and persistence of cognitive dysfunction, particularly following adolescent inhalant abuse, remain equivocal.

OBJECTIVE

The present study assessed specific cognitive processes beginning in late adolescence and adulthood following adolescent inhalation of toluene, a main component of many compounds readily abused.

METHODS

Adolescent male Wistar rats (postnatal day (PN) 27) were exposed to chronic intermittent inhaled toluene (10,000 ppm) for 1 h/day, 3 days/week for 4 weeks (PN 27-52) to mimic the patterns observed in human adolescent inhalant abusers. Following toluene exposure, motor and cognitive function was assessed.

RESULTS

Adolescent toluene exposure did not alter motor learning in the Rotarod task (PN 58) or acquisition, reversal, or retention of spatial learning in the Morris water maze (PN 55-64). In contrast, it delayed acquisition of instrumental responding for sucrose (5 % w/v) and impaired operant reversal learning and cue-induced reinstatement of sucrose seeking in adulthood (PN 57-100).

CONCLUSION

This study demonstrates that exposure to toluene at an abuse concentration during adolescence results in specific impairments in aspects of instrumental learning, without altering motor function and spatial learning in late adolescence/early adulthood. Our data imply that persistent alterations in reward processing may occur following adolescent inhalant misuse.

Review of toluene action: clinical evidence, animal studies and molecular targets

It has long been known that individuals will engage in voluntary inhalation of volatile solvents for their rewarding effects. However, research into the neurobiology of these agents has lagged behind that of more commonly used drugs of abuse such as psychostimulants, alcohol and nicotine. This imbalance has begun to shift in recent years as the serious effects of abused inhalants, especially among children and adolescents, on brain function and behavior have become appreciated and scientifically documented. In this review, we discuss the physicochemical and pharmacological properties of toluene, a representative member of a large class of organic solvents commonly used as inhalants. This is followed by a brief summary of the clinical and pre-clinical evidence showing that toluene and related solvents produce significant effects on brain structures and processes involved in the rewarding aspects of drugs. This is highlighted by tables highlighting toluene's effect on behaviors (reward, motor effects, learning, etc.) and cellular proteins (e.g. voltage and ligand-gated ion channels) closely associated the actions of abused substances. These sections demonstrate not only the significant progress that has been made in understanding the neurobiological basis for solvent abuse but also reveal the challenges that remain in developing a coherent understanding of this often overlooked class of drugs of abuse.

Indoor volatile organic compounds and chemical sensitivity reactions

Studies of unexplained symptoms observed in chemically sensitive subjects have increased the awareness of the relationship between neurological and immunological diseases due to exposure to volatile organic compounds (VOCs). However, there is no direct evidence that links exposure to low doses of VOCs and neurological and immunological dysfunction. We review animal model data to clarify the role of VOCs in neuroimmune interactions and discuss our recent studies that show a relationship between chronic exposure of C3H mice to low levels of formaldehyde and the induction of neural and immune dysfunction. We also consider the possible mechanisms by which VOC exposure can induce the symptoms presenting in patients with a multiple chemical sensitivity.

Solvents and Parkinson disease: a systematic review of toxicological and epidemiological evidence

Parkinson disease (PD) is a debilitating neurodegenerative motor disorder, with its motor symptoms largely attributable to loss of dopaminergic neurons in the substantia nigra. The causes of PD remain poorly understood, although environmental toxicants may play etiologic roles. Solvents are widespread neurotoxicants present in the workplace and ambient environment. Case reports of parkinsonism, including PD, have been associated with exposures to various solvents, most notably trichloroethylene (TCE). Animal toxicology studies have been conducted on various organic solvents, with some, including TCE, demonstrating potential for inducing nigral system damage. However, a confirmed animal model of solvent-induced PD has not been developed. Numerous epidemiologic studies have investigated potential links between solvents and PD, yielding mostly null or weak associations. An exception is a recent study of twins indicating possible etiologic relations with TCE and other chlorinated solvents, although findings were based on small numbers, and dose-response gradients were not observed. At present, there is no consistent evidence from either the toxicological or epidemiologic perspective that any specific solvent or class of solvents is a cause of PD. Future toxicological research that addresses mechanisms of nigral damage from TCE and its metabolites, with exposure routes and doses relevant to human exposures, is recommended. Improvements in epidemiologic research, especially with regard to quantitative characterization of long-term exposures to specific solvents, are needed to advance scientific knowledge on this topic.

Subclinical abnormalities in workers with continuous low-level toluene exposure

Short-term exposure to a high concentration (TWA > 100 ppm) of toluene can cause hepatotocixity and neurotoxicity in humans. Data on the effects of exposure to low levels of toluene, however, are controversial. In addition, few studies on the effects of toluene exposure on the autonomic nervous system have been conducted. Urine samples from 34 male factory workers in Taiwan who were exposed to low levels of toluene either intermittently (n = 13) or continuously (n = 21) were taken on a Monday morning after a 2-day hiatus and at the end of the workweek on Friday evening. Urinary hippuric acid levels were measured using high-performance liquid chromatography (HPLC). A complete blood work-up was also performed for each subject. The prevalence and severity of neurotoxic symptoms were investigated by a self-reported questionnaire, a neuropsychiatric battery, and sympathetic and peripheral nerve function tests. The mean value of urinary hippuric acid corrected for creatinine (Cr) was 0.34 ± 0.18 g/g Cr on Monday morning and 0.43 ± 0.26 g/g Cr on Friday evening. The difference in the mean value of urinary hippuric acid between the two periods (p < 0.01) and the odds ratio of impairment of sympathetic (OR = 4.13, p = 0.11) and peripheral nerves (OR = 6.94, p = 0.074) were higher in workers continuously exposed to toluene. In addition, workers who were continuously exposed to toluene had a lower mean platelet count (216 ± 41 × 10(6) /µL) than workers who were intermittently exposed (252 ± 40 × 10(6)/µL), (p = 0.018). Furthermore, there was a positive relationship between neurological abnormalities and a self-reported neuropsychiatric measurement (r = 0.35-0.66, p < 0.05) in all workers. These data suggest that continuous exposure to low levels of toluene may be associated with sympathetic and peripheral nerve dysfunction and sub-clinical hematological damage. Further research needs to be carried out regarding how chronic exposure to low-levels of toluene affects workers.

Neurotoxicity of toluene

Chemical susceptibility is triggered by a large range of chemicals present both indoors and outdoors including pesticides, cleaning products, perfumes, scented products and cigarette smoke. Health risk after chemical exposure depends on age, sex, genetic factors, socioeconomic status, nutritional status, and environmental factors. Toluene is one of volatile organic chemicals that causes different sensitivity in individuals. Although neurotoxic and immunotoxic effects of toluene have been studied extensively, the underlying mechanism remains obscure. This review highlights the possible neuroimmune factors influencing toluene sensitivity and neurotoxicity in a mouse model.

Contrasting effects of 4-week inhalation exposure to pseudocumene or hemimellitene on sensitivity to amphetamine and propensity to amphetamine sensitization in the rat

OBJECTIVES

Some data suggest that increased behavioural sensitivity to psychostimulants may develop after exposure to volatile chemicals in common use. The purpose of the present experiment was to find out whether and in what way inhalation exposure to pseudocumene (PS) or hemimellitene (HM) at low concentrations alters behavioural sensitivity to the psychostimulant amphetamine (AMPH), and propensity to develop behavioural sensitization to AMPH.

MATERIAL AND METHODS

Adult male Wistar rats were exposed 6 h/day, 5 days a week for 4 weeks to PS or HEM at 0, 25, 100 or 250 ppm. Behavioural sensitivity to AMPH was assessed by measuring locomotor activity of the animals in an open-field. Behavioural sensitization to AMPH was induced by a repeated AMPH treatment.

RESULTS

In rats exposed to HEM, the behavioural sensitivity to AMPH was increased, but remained unchanged in rats exposed to PS. The second testing revealed an augmented behavioural response to AMPH in control rats. In the HM exposed rats this augmenting was significantly more evident and in the PS exposed rats significantly less evident than in controls. For each of the two solvents, the concentration-effect relationship was nonlinear; out of the three concentrations used, 100 ppm was the most effective.

CONCLUSIONS

The results confirm that low-level inhalation exposure to trimethylbenzene isomers may induce behavioural sensitisation and/or increase the susceptibility of the animals to develop this state upon repeated psychostimulant treatment. They show, however, that HM and PS differ markedly in their ability to induce such alterations.

Up-regulation of neurotrophin-related gene expression in mouse hippocampus following low-level toluene exposure

To investigate the role of strain differences in sensitivity to low-level toluene exposure on neurotrophins and their receptor levels in the mouse hippocampus, 8-week-old male C3H/HeN, BALB/c and C57BL/10 mice were exposed to 0, 5, 50, or 500 ppm toluene for 6h per day, 5 days per week for 6 weeks in an inhalation chamber. We examined the expressions of neurotrophin-related genes and receptors in the mouse hippocampus using real-time reverse transcription polymerase chain reaction (RT-PCR). The expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tyrosine kinase (Trk) A, and TrkB mRNAs in the C3H/HeN mice hippocampus was significantly higher in the mice exposed to 500 ppm toluene. Among the three strains of mice, the C3H/HeN mice seemed to be sensitive to toluene exposure. To examine the combined effect of toluene exposure and allergic challenge, the C3H/HeN mice stimulated with ovalbumin were exposed to toluene. The allergy group of C3H/HeN mice showed significantly elevated level of NGF mRNA in the hippocampus following exposure to 50 ppm toluene. Then, we also examined the expression of transcription factor, dopamine markers and oxidative stress marker in the hippocampus of sensitive strain C3H/HeN mice and found that the expression of CREB1 mRNA was significantly increased at 50 ppm toluene. In immunohistochemical analysis, the density of the NGF-immunoreactive signal was significantly stronger in the hippocampal CA3 region of the C3H/HeN mice exposed to 500 ppm toluene in non-allergy group and 50 ppm in allergy group. Our results indicate that low-level toluene exposure may induce up-regulation of neurotrophin-related gene expression in the mouse hippocampus depending on the mouse strain and an allergic stimulation in sensitive strain may decrease the threshold for sensitivity at lower exposure level.

Athymic nude mice are insensitive to low-level toluene-induced up-regulation of memory-related gene expressions in the hippocampus

The function of the N-methyl-d-aspartate (NMDA) subtype of glutamatergic receptors is known to be antagonized by toluene, a well-characterized neurotoxic chemical known to impair memory functions. Recently, peripheral T cells have been clearly shown to play an important role in cognitive and behavioral functions. In the present study, we investigated the role of peripheral T cells in the hippocampal mRNA expression of memory-related genes induced by low levels of toluene exposure in mice. BALB/c wild-type (WT) and nude mice were exposed to 9ppm of toluene or filtered air (0ppm toluene; control groups) in a nose-only exposure chamber for 30min on 3 consecutive days followed by weekly sessions for 4 weeks. Twenty-four hours after the last exposure, the hippocampi were collected and the inducibility of memory-related genes was examined using a real-time quantitative PCR method. NMDA NR2A, calcium/calmodulin-dependent protein kinase IV (CaMKIV), cyclic AMP-responsive element binding protein 1 (CREB1), and BDNF were significantly up-regulated in the hippocampi of WT mice exposed to 9ppm of toluene, compared to the expressions observed in WT mice exposed to filtered air, but similar results were not observed in nude mice. To investigate the possible involvement of peripheral T cells in the toluene-induced up-regulation of memory-related genes in WT mice, we examined the mRNA expression of Thy-1 (a pan T cell-specific marker) and quantified the number of cells that were immunoreactive to a T cell antigen receptor, CD3 (CD3-ir). Both the expression of Thy-1 mRNA and the number of CD3-ir cells were significantly higher in the hippocampi of the WT mice exposed to 9ppm of toluene, compared with that in WT mice exposed to filtered air; similar results were not observed in nude mice. We also examined the expression of chemokine genes like CCL2 and CCL3. The expression of CCL3 mRNA was significantly up-regulated only in the toluene-exposed WT mice. Although other differences unrelated to immune function may exist between WT and nude mice from the same background, the findings of the present study strongly suggest that the recruitment of peripheral T cells in the hippocampi of BALB/c WT mice exposed to low levels of toluene may be involved in the toluene-induced up-regulation of memory-related genes at the mRNA level.

Modulation of neurological related allergic reaction in mice exposed to low-level toluene

The contributing role of indoor air pollution to the development of allergic disease has become increasingly evident in public health problems. It has been reported that extensive communication exists between neurons and immune cells, and neurotrophins are molecules potentially responsible for regulating and controlling this neuroimmune crosstalk. The adverse effects of volatile organic compounds which are main indoor pollutants on induction or augmentation of neuroimmune interaction have not been fully characterized yet. To investigate the effects of low-level toluene inhalation on the airway inflammatory responses, male C3H mice were exposed to filtered air (control), 9 ppm, and 90 ppm toluene for 30 min by nose-only inhalation on Days 0, 1, 2, 7, 14, 21, and 28. Some groups of mice were injected with ovalbumin intraperitoneally before starting exposure schedule and these mice were then challenged with aerosolized ovalbumin as booster dose. For analysis of airway inflammation, bronchoalveolar lavage (BAL) fluid were collected to determine inflammatory cell influx and lung tissue and blood samples were collected to determine cytokine and neurotrophin mRNA and protein expressions and plasma antibody titers using real-time RT-PCR and ELISA methods respectively. Exposure of the ovalbumin-immunized mice to low-level toluene resulted in (1) increased inflammatory cells infiltration in BAL fluid; (2) increased IL-5 mRNA, decreased nerve growth factor receptor tropomyosin-related kinase A and brain-derived neurotrophic factor mRNAs in lung; and (3) increased IgE and IgG(1) antibodies and nerve growth factor content in the plasma. These findings suggest that low-level toluene exposure aggravates the airway inflammatory responses in ovalbumin-immunized mice by modulating neuroimmune crosstalk.

Effect of long-term exposure to low-level toluene on airway inflammatory response in mice

Volatile organic compounds are the main substances causing multiple chemical sensitivity reactions in human. Our laboratory has previously showed that the exposure of low-level formaldehyde causes immunogenic and neurogenic inflammatory responses in mice. The aim of the present study was to investigate the effect of long-term, low-level toluene exposure on airway inflammatory responses in mice lung. We exposed female C3H mice to filtered air (0ppm) or 50ppm of toluene for 6h/day on 5days/week for 6 or 12 weeks in the whole body exposure chamber. One day following the last toluene exposure, we collected bronchoalveolar lavage fluid from each mouse and examined cellular infiltration and production of cytokines, chemokines, neurotrophins and substance P by using ELISA method. We found that the number of total cells and macrophages increased significantly in both 6 and 12-week-exposed mice. In addition, the production of interferon-gamma and substance P were decreased significantly and nerve growth factor was not affected in both 6 and 12-week-exposed mice. In contrast, neurotrophin-3 production in bronchoalveolar lavage fluid was significantly increased only in 12-week-exposed mice. Our findings suggest that long-term (12-week) exposure of mice to low-level toluene modulates airway inflammatory response via neurological signaling.

Toluene induces rapid and reversible rise of hippocampal glutamate and taurine neurotransmitter levels in mice

Toluene, a widely used aromatic organic solvent, has been well characterized as a neurotoxic chemical. Although the neurobehavioral effects of toluene have been studied substantially, the mechanisms involved are not clearly understood. Hippocampus, which is one of the limbic areas of brain associated with neuronal plasticity, and learning and memory functions, may be a principal target of toluene. In the present study, to establish a mouse model for investigating the effects of acute toluene exposure on the amino acid neurotransmitter levels in the hippocampus, in vivo microdialysis study was performed in freely moving mice after a single intraperitoneal administration of toluene (150 and 300 mg/kg). Amino acid neurotransmitters in microdialysates were measured by a high performance liquid chromatography system. The extracellular levels of glutamate and taurine were rapidly and reversibly increased within 30 min after the toluene administration in a dose-dependent manner and returned to the basal level by 1h. Conversely, the extracellular level of glycine and GABA were stable, and no significant change was observed after the toluene administration. To further investigate the brain toluene level in the hippocampus of toluene-administered mice, we used a solid-phase microextraction (SPME) method and examined the time course changes of toluene in the hippocampus of living mice. The brain toluene level reached the peak at 30 min after injection and returned to the basal level after 2h. In the present study, we observed the relationship between brain toluene levels and amino acid neurotransmitter glutamate and taurine levels in the hippocampus. Therefore, we suggest that toluene may mediate its action through the glutamatergic and taurinergic neurotransmission in the hippocampus of freely moving mice.

Long-lasting neurotoxicity of prenatal benzene acute exposure in rats

Benzene is a common element of environmental pollution. Although this substance is not recognized as a teratogenic agent, it is not known whether prenatal exposure to benzene may induce neurobehavioral changes in the progeny. Benzene 0.1mg/kg was injected subcutaneously (s.c.) acutely at day 15 of gestation into pregnant female rats of the Sprague-Dawley strain and neurotoxicity of the substance was studied in pups and male adult animals of the same progeny. No change was found in total number of neonates, body weight and eye opening time between benzene-exposed animals and controls. No malformations were observed. At birth, neonatal reflexes (cliff aversion, forelimb placing, bar holding, forelimb grasping, startle) were scored in benzene-exposed pups and their percent appearance was found to be anticipated (more benzene-exposed pups exhibited reflexes each day) in comparison to that of control animals. Also, the completion (maximum appearance, i.e. 100% of the brood was found to exhibit each reflex) of neonatal reflexes in benzene-exposed animals preceded that of controls. Starting 2 months after birth, cognitive and motor performance was assessed only in male animals of the prenatally benzene-exposed progeny. The overall evaluation of motor activity in benzene-exposed animals in the open-field test revealed reduced ambulation in these rats as compared to control animals. Acquisition of active avoidance responses in the shuttle-box test, as assessed by the number of conditioned avoidance responses and the percent of learners, was impaired in benzene-exposed rats as compared to control animals. Prenatal exposure to benzene was also followed by reduced retention latency in a step-through passive avoidance task in two retention tests. These results suggest that acute exposure to benzene during gestational organogenesis may cause long-lasting changes in motor behavior and cognitive processes. This may be relevant for the assessment of benzene toxic profile for the progeny of pregnant subjects, although teratogenic effects are not observed.

Learning and memory deficits in rats induced by chronic thinner exposure are reversed by melatonin

Thinner is a neurotoxic mixture which is widely used as an aromatic industrial solvent. This product has been shown to cause functional and structural changes in the central nervous system. We investigated the effect of exposure to high concentrations (3000 p.p.m.) of thinner for 45 days (1 hr/day) on cognitive functions and the levels of neural cell adhesion molecules (NCAM) and lipid peroxidation products (LPO) in the hippocampus, cortex and cerebellum of rats. The actions of melatonin on the effects produced by thinner exposure were also tested. Thinner exposure caused a significant increase in LPO (malondialdehyde and 4-hydroxyalkenals) in all brain regions. Melatonin administration significantly reduced LPO and elevated glutathione levels in the brain regions. NCAM (180 kDa) was significantly decreased in hippocampus and cortex of thinner-exposed rats. Furthermore, thinner-exposed rats showed cognitive deficits in passive avoidance and Morris water maze tasks, whereas in the rats chronically treated with melatonin these effects were reversed. This study indicates that treatment with melatonin prevents learning and memory deficits caused by thinner exposure possibly by reducing oxidative stress and regulating neural plasticity.

Hypothalamo-pituitary-adrenal gland axis in mice inhaling toluene prior to low-level long-term exposure to formaldehyde

We studied the change in the hypothalamo-pituitary-adrenal gland (HPA) axis upon adding prior toluene inhalation to our previous formaldehyde inhalation experiments to determine whether short term exposure to relatively high levels of toluene triggers multiple chemical sensitivity (MCS). Data come from immunocytochemical, morphometrical and RT-PCR measurements. Four groups of adult female mice were exposed to differing concentrations (0, 80, 400, and 2,000 ppb) of formaldehyde for 16 hr/day, 5 days/week for twelve weeks, after the mice were exposed intranasally to 500 ppm toluene per mouse for 6 hr/day, for 3 days. We found that the number of corticotropin releasing hormone (CRH)-immunoreactive (ir) neurons was up-regulated according to the amount of formaldehyde as well as inhalation of formaldehyde alone in our previous experiment. The proportion of adrenocorticotropin hormone (ACTH)-ir cells increased according to the formaldehyde concentration, though there was no significant difference between the 400 and 2,000 groups. The number of ACTH-ir cells was higher in the 400 group than in the other groups (0, 80, and 2,000). Expression of ACTH-mRNA was also up-regulated according to the quantity of formaldehyde. The sinusoid in the anterior pituitary showed more dilatation in the 400 and 2,000 groups than in the control group, especially in the 2,000 group. We propose that exposure to toluene prior to inhalation of formaldehyde has no effect on the HPA axis and as a trigger of MCS, although greater sinusoid dilatation was found in the anterior pituitary gland at higher concentrations of formaldehyde.

Effects of thinner exposure on the expression pattern of neural cell adhesion molecules, level of lipid peroxidation in the brain and cognitive function in rats

Thinner containing 60-70% toluene is a neurotoxic mixture, which is widely used as an aromatic industrial solvent. This product has been shown to cause functional and structural changes in the central nervous system. Thinner generates reactive oxygen species and the toxic effects relating to these reactants. We have investigated the effect of exposure to high concentrations (3000 ppm) of thinner for 45 days (1 h/day) on cognitive function and the levels of neural cell adhesion molecules (NCAM) and lipid peroxidation products in the hippocampus, cortex and cerebellum of rats. Thinner exposure caused a significant increase in lipid peroxidation products (malondialdehyde and 4-hydroxyalkenals) in all brain regions. Levels of NCAM 140 and NCAM 180 kDa were significantly decreased in the hippocampus and cortex of the thinner-exposed group. Furthermore, thinner-exposed rats showed cognitive deficits in passive avoidance and Morris water maze tasks. These cognitive deficits may be due to both elevated oxidative stress and changes in synaptic plasticity. Almost all studied parameters were reversed in animals which were allowed to recover from thinner inhalation, suggesting that the effects of exposure to thinner, at least for 1 h/day for 45 days, are reversible.

Repeated exposure to the abused inhalant toluene alters levels of neurotransmitters and generates peroxynitrite in nigrostriatal and mesolimbic nuclei in rat

Toluene, a volatile hydrocarbon found in a variety of chemical compounds, is misused and abused by inhalation for its euphorigenic effects. Toluene's reinforcing properties may share a common characteristic with other drugs of abuse, namely, activation of the mesolimbic dopamine system. Prior studies in our laboratory found that acutely inhaled toluene activated midbrain dopamine neurons in the rat. Moreover, single systemic injections of toluene in rats produced a dose-dependent increase in locomotor activity which was blocked by depletion of nucleus accumbens dopamine or by pretreatment with a D2 dopamine receptor antagonist. Here we examined the effects of seven daily intraperitoneal injections of 600 mg/kg toluene on the content of serotonin and dopamine in the caudate nucleus (CN) and nucleus accumbens (NAC), substantia nigra, and ventral tegmental area at 2, 4, and 24 h after the last injection. Also, the roles of nitric oxide, peroxynitrite, and the production of 3-nitrosotyrosine (3-NT), in the CN and NAC were assessed at the same time points. Toluene treatments increased dopamine levels in the CN and NAC, and serotonin levels in CN, NAC, and ventral tegmental area. Measurements of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC) further suggested a change in transmitter utilization in CN and NAC. Lastly, 3-NT levels also showed a differential change between CN and NAC, but at different time points post-toluene injection. These results point out the complexity of action of toluene on neurotransmitter function following a course of chronic exposure. Changes in the production of 3-NT also suggest that toluene-induced neurotoxicity may mediate via generation of peroxynitrite.

Metabolite alterations in basal ganglia associated with psychiatric symptoms of abstinent toluene users: a proton MRS study

Long-term toluene abuse causes a variety of psychiatric symptoms. However, little is known about abnormalities at the neurochemical level in the living human brain after long-term exposure to toluene. To detect neurochemical changes in the basal ganglia of subjects with a history of long-term toluene use, proton magnetic resonance spectroscopy (1H MRS) was performed in 12 abstinent toluene users and 13 healthy comparisons with no history of drug abuse. N-acetylaspartate (NAA), creatine plus phosphocreatine (Cr + PCr), choline-containing compounds (Cho), and myo-inositol (MI) levels were measured in the left and right basal ganglia. The Cho/Cr + PCr ratio, a marker of membrane metabolism, was significantly increased in the basal ganglia of toluene users in comparison to that of the control subjects. Furthermore, the increase in the Cho/Cr + PCr ratio was significantly correlated with the severity of residual psychiatric symptoms. These findings suggest that long-term toluene use causes membrane disturbance in the basal ganglia, which is associated with residual psychiatric symptoms that persist even after long-term abstinence from toluene use.

Sub-chronic exposure to toluene at 40ppm alters the monoamine biosynthesis rate in discrete brain areas

Effects of long-term exposure to a sub-toxic concentration of toluene (40 ppm, 104 h per week, 16 weeks) have been studied on monoamine biosynthesis rate in rat. The activities of the rate limiting enzymes in catecholamine and 5-hydroxytryptamine biosynthesis tyrosine and tryptophan hydroxylase, respectively, were estimated in vivo by measuring the accumulation of l-dihydroxyphenylalanine and 5-hydroxytryptophan after pharmacological blockade of l-aromatic acid decarboxylases by NSD-1015 (100 mg kg(-1) ip). The sub-chronic exposure to toluene led to a significant and gender dependent alteration in both catecholamine and 5-hydroxytryptamine biosynthesis rate in brainstem catecholaminergic cell groups and hypothalamus. In females exposed to toluene, changes in tryptophan hydroxylation were found in rostral subset of A2C2 (+41%) and in A1C1 (+41%) while a decrease in A5 (-44%) and an increase in A2C2 (+28%) were found in tyrosine hydroxylation. In males, an increase in tryptophane hydroxylation was observed in rostral subset of A2C2 (+35%) while a decrease was observed in ventro-median hypothalamus (-17%). These results suggest that toluene exposure to a dose generally recognized as sub-toxic (40 ppm, no observed adverse effect level) leads to adverse effects on monoaminergic systems. Therefore, the neurotoxicity of toluene should be carefully re-evaluated taking into account not only the exposure level but also the duration.

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

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

Behavioral and neurochemical effects induced by subchronic exposure to 40 ppm toluene in rats

Chronic toluene inhalation at concentrations above occupational exposure limits (e.g., 100 ppm; NIOSH) has been repeatedly shown to induce neurotoxic effects. In contrast, although few clinical and experimental data are available on the effects of toluene exposure at concentrations below occupational exposure standards, some of these data may support adverse effects of long-term exposure to low toluene concentrations. To test this hypothesis, we investigated the neurobehavioral and neurochemical effects of 40 ppm inhaled toluene in a rat model of 16-week subchronic exposure, examining locomotor and rearing activities; adaptation/sensitization to narcosis produced by acute exposure to toluene at high concentration; and tyrosine hydroxylase and tryptophan hydroxylase activities, and dopamine (DA) and serotonin (5-HT) turnovers in the caudate-putamen, nucleus accumbens, hippocampus, prefrontal cortex, and cerebellum. Our results mainly show that subchronic exposure to 40 ppm toluene significantly resulted in a sensitization to toluene-induced narcosis, a decrease in rearing activity, and alterations in DA and 5-HT transmissions. This demonstrates that subchronic toluene exposure at a low concentration may lead to adverse changes in neurobehavioral and neurochemical functioning, and further questions in a public health perspective the actual neurotoxic potential of toluene and other organic compounds, because deficits in functioning are generally viewed as precursors of more serious adverse effects.

Sensitization as a mechanism for multiple chemical sensitivity: relationship to evolutionary theory

Multiple chemical sensitivity (MCS) is a disorder in humans attributed to prior chemical exposure. Sensitization is an amplification of neuronal responsiveness that elicits increased behavioral responding to stimuli, and occurs in a recently developed rat model of MCS. Rats were exposed to repeated formaldehyde (Form) and their response in three behavioral tests, including locomotor activity after a cocaine challenge, conditioned fear, and behavioral avoidance of Form, was assessed. In all three tests, rats demonstrated sensitized behaviors, implicating amplified responding within specific limbic brain regions. Evolutionary theory in the context of MCS specifies how the behavioral strategies of those with MCS are consistent with the notion that their self-perceived sense of survival and reproductive fitness may be threatened by chemical exposures. This behavior may be mediated by the same limbic brain regions that become sensitized after repeated chemical exposure in animals.

Toluene inhalation produces regionally specific changes in extracellular dopamine

The aim of the present study was to investigate the effect of toluene inhalation on dopaminergic transmission in two distinct brain areas presumably involved in mediating the reward processes important for toluene abuse. Extracellular dopamine (DA) levels were measured in prefrontal cortex (PFC) and nucleus accumbens (NACC) of freely moving rats using in vivo microdialysis. Inhalation of a behaviorally relevant concentration of toluene (3000 ppm) produced a significant increase in the PFC but not in the NACC. However, the odorant isoamyl acetate, increased PFC DA levels by only 37%, significantly less than the 96% increase observed following toluene exposure. When toluene inhalation was combined with cocaine administration (20 mg/kg i.p.), the response to the combined challenge was not different from the response to toluene alone in the PFC. However, the combination of these two drugs produced a supradditive response of 802% in the NACC, compared with the 450% increase observed following cocaine alone. Recent reports indicate that toluene influences the function of several ionotropic receptors in a subunit specific manner. As further evidence of specific effects, our results indicate regionally specific changes in dopaminergic transmission following toluene exposure.

Effects of aerosol-vapor JP-8 jet fuel on the functional observational battery, and learning and memory in the rat

To determine whether JP-8 jet fuel affects parameters of the Functional Observational Battery (FOB), visual discrimination, or spatial learning and memory, the authors exposed groups of male Fischer Brown Norway hybrid rats for 28 d to aerosol/vapor-delivered JP-8, or to JP-8 followed by 15 min of aerosolized substance P analogue, or to sham-confined fresh room air. Behavioral testing was accomplished with the U.S. Environmental Protection Agency's Functional Observational Battery. The authors used the Morris swim task to test visual and spatial learning and memory testing. The spatial test included examination of memory for the original target location following 15 d of JP-8 exposure, as well as a 3-d new target location learning paradigm implemented the day that followed the final day of exposure. Only JP-8 exposed animals had significant weight loss by the 2nd week of exposure compared with JP-8 with substance P and control rats; this finding compares with those of prior studies of JP-8 jet fuel. Rats exposed to JP-8 with or without substance P exhibited significantly greater rearing and less grooming behavior over time than did controls during Functional Observational Battery open-field testing. Exposed rats also swam significantly faster than controls during the new target location training and testing, thus supporting the increased activity noted during Functional Observational Battery testing. There were no significant differences between the exposed and control groups' performances during acquisition, retention, or learning of the new platform location in either the visual discrimination or spatial version of the Morris swim task. The data suggest that although visual discrimination and spatial learning and memory were not disrupted by JP-8 exposure, arousal indices and activity measures were distinctly different in these animals.

Use of positron emission tomography in analysing receptor function in vivo

The non-invasive radiotracer technique positron emission tomography (PET) may provide valuable information in the toxicokinetic-toxicodynamic evaluation of endogenous or toxic environmental compounds. Assessment of mechanism of action of toxins is often difficult to validate. In this respect, PET may offer advantages since it can quantify not only the distribution and kinetics of the radiolabelled toxin in the body, but also the altered rates of physiological or biochemical processes induced by the toxin. It is even possible to validate the body distribution and tissue accumulation of the toxic compound in primates, since linear kinetics can be assumed after administration of the radiolabelled compound in minute amounts without any toxic or physiological effects. Quantitative estimates can be derived with accuracy and high precision. Using a multi-tracer protocol, it is often possible to illuminate both the kinetics and the dynamics of a toxic compound. Long-term effects of different toxins on dopamine receptor function have been evaluated with PET as well as the influence of Parkinson disease medication on pre- and postsynaptic dopaminergic receptor function over the course of the disease. In conclusion, PET may provide very informative insight into complex receptor interactions of both toxic compounds and drugs under development.

Repeated formaldehyde effects in an animal model for multiple chemical sensitivity

Chemical intolerance is a phenomenon observed in multiple chemical sensitivity (MCS) syndrome, an ill-defined disorder in humans attributed to exposure to volatile organic compounds. Amplification of symptoms in individuals with MCS resembles the phenomenon of psychostimulant- and stress-induced sensitization in rodents. We have recently tested in rats the hypothesis that repeated chemical exposure produces sensitization of central nervous system (CNS) circuitry. A rat model of MCS in our laboratory has employed several endpoints of CNS function after repeated formaldehyde (Form) exposure (1 h/day x 5 days/week x 4 weeks). Repeated Form exposure produced behavioral sensitization to later cocaine injection, suggesting altered dopaminergic sensitivity in mesolimbic pathways. Rats given repeated Form also demonstrated increased fear conditioning to odor paired with footshock, implicating amplification of neural circuitry guiding fear responding to a conditioned odor cue. Recent studies examining the effects of repeated Form on locomotor activity during each daily exposure showed a decrease in rearing activity after 12-15 days of Form exposure compared to air-exposed controls. EEG recordings taken 1 week after withdrawal from daily Form revealed altered sleep architecture. Some of the differences in sleep disappeared after subsequent brief (15 min) challenge with Form the next day. Overall, the findings indicate that repeated low-level chemical exposure produces behavioral changes that may be akin to those observed in individuals with MCS, such as greater sensitivity to chemicals manifest as increased anxiety upon chemical exposure and altered sleep and/or fatigue. Study of the underlying CNS changes will provide a basis for mechanistically based animal models for MCS.

Behavioral effects following subacute inhalation exposure to m-xylene or trimethylbenzene in the rat: a comparative study

Trimethylbenzene (TMB), like xylene (dimethylbenzene), is a significant constituent of some industrial solvent mixtures. In earlier studies, we found that in the rat a subacute low-level inhalation exposure to some of the TMB isomers may result in behavioral alterations detectable weeks after the exposure [Neurotoxicol Teratol 19;1997:327; Int J Occup Med Environ Health 11;1998:319]. The purpose of the present study was to compare m-xylene (XYL) and each of the TMB isomers: 1,2,3-TMB (hemimellitene - HM), 1,2,4-TMB (pseudocumene - PS), and 1,3,5-TMB (mesitylene - MES) with respect to the ability for inducing behavioral effects in the rat. The rats (10-11 animals per group) were exposed repeatedly for 4 weeks (6 h per day, 5 days per week) to XYL (XYL group), HM (HM group), PS (PS group) or MES (MES group) at 100 ppm, or sham exposed (C group) in 1.3 cu/m dynamic inhalation chambers. Starting 2 weeks after exposure the following forms of rat's behavior were assessed: radial maze performance, spontaneous activity in an open field, learning and retention of passive and active (two-way) avoidance response, and heat-induced paw licking before and after a 2 min footshock (a test for assessment of the stress response). None of the solvent-exposed groups differed considerably from the control one with respect to the radial maze performance. Compared to control rats, the rats of the XYL, PS and MES groups, but not those of HM group, showed a significantly higher spontaneous locomotor activity in the open field, an impaired passive avoidance learning and significantly longer paw-lick latencies 24 h after footshock. Acquisition, but not retention, of the two-way active avoidance response was significantly impaired in all solvent-exposed groups. The XYL group did not differ significantly from PS, MES or HM group in any of the behavioral parameters. The above results show that a short-term exposure to any of the TMB isomers or m-xylene at concentration as low as 100 ppm may induce persistent behavioral alterations in the rat.

Inhalation of low concentrations of toluene induces persistent effects on a learning retention task, beam-walk performance, and cerebrocortical size in the rat

The organic solvent toluene is widely used in industry. The threshold limit value for extended occupational exposure to toluene is presently set to 200 ppm in the United States. We have investigated the effect of an inhalation exposure of 80 ppm for 4 weeks (6 h/day, 5 days/week), followed by a postexposure period of at least 4 weeks, on behavior and brain features in the rat. Toluene exposure appeared to affect spatial memory, since toluene-exposed rats showed a longer time in the correct quadrant in a Morris swim maze. This effect may indicate that the exposed rats used their praxis strategy longer before they started to look for the platform elsewhere. Toluene-exposed rats showed trends for increases in both locomotion and rearing behaviors and a significantly reduced beam-walk performance. The area of the cerebral cortex, especially the parietal cortex, was decreased by 6-10% in toluene-exposed rats, as shown by magnetic resonance imaging of living rats and autoradiograms of frozen brain sections. The K(D) and B(max) values of the dopamine D(3) agonist [(3)H]PD 128907 were not affected by toluene, as measured in caudate-putamen and subcortical limbic area using biochemical receptor binding assays and in caudate-putamen and islands of Calleja using quantitative receptor autoradiography. Hence, previously demonstrated persistent effects by toluene on the binding characteristics of radioligands binding to both D(2) and D(3) receptors seem to indicate a persistent effect of toluene selectively on dopamine D(2) receptors. Taken together, the present results indicate that exposure to low concentrations of toluene leads to persistent effects on cognitive, neurological, and brain-structural properties in the rat.

Subchronic toluene exposure in low concentrations produces signs of reduced dysfunction in the 6-hydroxydopamine lesioned nigrostriatal dopaminergic system of the rat

The effect of a subchronic (4-week) exposure to low concentrations of toluene (40 or 80 parts per million, ppm) on the brain dopaminergic system has been examined in a rat model of Parkinson's disease. A unilateral lesion of the substantia nigra (SN) dopamine (DA) nerve cells was performed by injection of a low dose of 6-hydroxydopamine (6-OH DA). The peak activity of contralateral rotational behavior induced by apomorphine was significantly decreased after exposure to 80 ppm toluene. Analysis of the neostriatum and SN ipsilateral to the lesion revealed that toluene (80 ppm, but not 40 ppm) counteracted the 6-OH DA-induced reductions of DA tissue levels both within the SN and the neostriatum. Also the lesion-induced reduction of immunoreactivity for tyrosine hydroxylase (TH IR) in the neostriatum was partly counteracted by the toluene exposure (80 ppm). In conclusion, a subchronic exposure to low doses of toluene (80 ppm) leads to signs of reduced dysfunction of the nigrostriatal dopaminergic system after the neurotoxic treatment.

Behavioral sensitization after repeated formaldehyde exposure in rats

Multiple chemical sensitivity (MCS) is a phenomenon whereby individuals report increased sensitivity to chemicals in the environment, and attribute their sensitivities to prior exposure to the same or often structurally unrelated chemicals. A leading hypothesis suggests that MCS is akin to behavioral sensitization observed in rodents after repeated exposure to drugs of abuse or environmental stressors. Sensitization occurring within limbic circuitry of the central nervous system (CNS) may explain the multisymptom complaints in individuals with MCS. The present studies represent the continuing development of an animal model for MCS, the basis of which is the CNS sensitization hypothesis. Three behaviors were assessed in rats repeatedly exposed to formaldehyde (Form) inhalation. In the first series of experiments, rats were given high-dose Form exposure (11 parts per million [ppm]; 1 h/day x 7 days) or low-dose Form exposure (1 ppm; either 1 h/day x 7 days or 1 h/day x 5 days/week x 4 weeks). Within a few days after discontinuing daily Form, cocaine-induced locomotor activity was elevated after high-dose Form or 20 days of low-dose Form inhalation. Approximately 1 month later, cocaine-induced locomotor activity remained significantly elevated in the 20-day Form-exposed rats. The second experiment assessed whether prior exposure to Form (20 days, as above) would alter the ability to condition to an odor (orange oil) paired with footshock. The results suggested a tendency to increase the conditioned fear response to the odor but not the context of the footshock box, and a decreased tendency to extinguish the conditioned fear response to odor. The third experiment examined whether CNS sensitization to daily cocaine or stress would alter subsequent avoidance responding to odor (Form). Daily cocaine significantly elevated approach responses to Form, while daily stress pretreatment produced a trend in the opposite direction, producing greater avoidance of Form. Preliminary studies indicated that repeated daily Form inhalation (20 days, as above) produced a greater avoidance to subsequent Form presentation, suggesting that daily Form inhalation may serve as a stressor. The results support the hypothesis that repeated chemical exposure in rats may produce CNS plasticity manifest as greater sensitivity to dopaminergic drugs, enhanced fear conditioning to odor paired with an aversive event, and greater avoidance of odors. Some of these behavioral changes observed in rats may provide a link with symptoms in a subset of individuals with MCS.

A rat model of neurobehavioral sensitization to toluene

Some individuals report that, following either a single high-level or repeated lower-level exposures to chemicals (initiation), subsequent exposure to very low concentrations of chemicals (triggering) produces a variety of adverse effects, including disruption of cognitive processes. Our objective was to model this two-step process in a laboratory animal. Two groups of 16 rats, eight male and eight female, received whole-body inhalation exposure to toluene, either at 80 ppm for 6 h/day for 4 weeks (Repeat group) or to 1600 ppm for 6 h/day on one day only (Acute group). Two other groups (Trigger group and Clean group) of 16 were sham-exposed. After 17 days without toluene exposure, the Acute, Repeat and Trigger groups began a series of daily toluene 'trigger' exposures (10 ppm for 1 h) followed immediately by testing on an operant repeated-acquisitions task requiring learning within and across sessions. The Clean group was sham-exposed prior to operant testing. Trigger or sham exposures and operant testing continued 5 days/week for 17 sessions. Analysis of variance revealed a variety of statistically significant (P < 0.05) differences between treatment groups. Furthermore, the patterns of differences between groups differed (P < 0.05) for female and male rats. For example, male rats of the Trigger group made the most responses, and female rats of the Repeat group responded most slowly. The observation of important changes in the operant behavior of female and male rats previously exposed to toluene, at relatively low concentrations (80 or 1600 ppm) and then later re-exposed at very low concentrations (10 ppm), is consistent with the experiences of humans reporting cognitive difficulties following acute or chronic exposures to chemicals.

Differential resting quantitative electroencephalographic alpha patterns in women with environmental chemical intolerance, depressives, and normals

BACKGROUND

Previous research suggests that a subset of individuals with intolerance to low levels of environmental chemicals have increased levels of premorbid and/or comorbid psychiatric disorders such as depression, anxiety, and somatization. The purpose of this study was to evaluate the psychological profiles and quantitative electroencephalographic (qEEG) profiles at baseline of women with and without chemical intolerance (CI).

METHODS

Participants were middle-aged women who reported illness from the odor of common chemicals (CI, n = 14), depressives without such intolerances (D, n = 10), and normal controls (N, n = 11). They completed a set of psychological scales and underwent two separate qEEG recording laboratory sessions spaced 1 week apart, at the same time of day for each subject.

RESULTS

CI were similar to D with increased lifetime histories of physician-diagnosed depression (71% vs. 100%), Symptom Checklist 90 (revised) (SCL-90-R) somatization scores, Barsky Somatic Symptom Amplification, and perceived life stressfulness, although D had more distress than either CI or N on several other SCL-90-R subscales. CI scored significantly higher on the McLean Limbic Symptom Checklist somatic symptom subscale than did either D or N. On qEEG, CI exhibited significantly greater overall resting absolute alpha activity with eyes closed, especially at the parietal midline site (Pz), and increased (sensitized) frontal alpha from session 1 to 2, in contrast with the D and N groups. D showed right frontal asymmetry in both sessions, in comparison with CI.

CONCLUSIONS

The data indicate that CI with affective distress diverge from both D without chemical intolerance and N in qEEG alpha patterns at resting baseline. Although CI descriptively resemble D with increased psychological distress, the CI's greater alpha suggests the possibility of a) central nervous system hypo-, not hyper-, activation; and/or b) an overlap with EEG alpha patterns of persons with positive family histories of alcoholism.

Proposed animal neurosensitization model for multiple chemical sensitivity in studies with formalin

A potentially promising line of animal research relevant to multiple chemical sensitivity (MCS) is that of sensitization in the central nervous system (CNS), particularly limbic pathways in the brain. Sensitization is the progressive and enduring enhancement in behavioral and neurochemical responses that occurs after repeated exposure to psychostimulants or environmental stressors. Since the onset and progression of sensitization has many parallels with that of MCS, it has been proposed that MCS may be initiated through a mechanism similar to the sensitization of CNS components occurring in the rodent. To test this hypothesis, female Sprague-Dawley rats were exposed to formalin vapors (FORM, 11 ppm) or water vapor (control) 1 h/day for 7 days. The next day, a saline injection was given followed by a cocaine injection (15 mg/kg, i.p.) 24 h later, and locomotor activity was monitored. Animals pretreated with repeated FORM inhalation demonstrated a significantly enhanced locomotor response to cocaine compared to controls, an indicator that specific limbic pathways may have been sensitized. At 4 weeks of withdrawal from FORM exposure, a subset of animals remained sensitized to a cocaine challenge. No differences were found between groups after a saline injection. In a second experiment, animals were screened prior to FORM or water exposure for their response to a novel situation, a measure believed to reflect an animal's general responsiveness to stimuli. Rats were divided into high responders (HR) or low responders (LR), based on their locomotion in a novel cage. Results from three behavioral tests demonstrated that HR and LR were differentially affected by exposure to FORM. In a passive avoidance test, HR and LR appeared to be different in their distribution of responses, while HR and LR responses in the FORM group were nearly identical. On the elevated plus maze test of anxiety, HR spent more time on the open arms than LR in both treatment groups, with significant differences between HR and LR in the FORM, but not water, treated group. On a hot plate test to measure nociceptive levels, no differences occurred between HR and LR in the control group, whereas nociception of LR tended toward an increase compared to HR in the FORM-exposed group. Results from the second experiment suggest that the effects of FORM exposure may be obscured by examining behavior in a heterogeneous population (HR and LR). This approach using animal models may help define neural substrates that mediate the amplification of responses of a subpopulation of individuals to chemicals in the environment.