Effects of toluene inhalation on detection of auditory signals in rats

Translational cognitive systems: focus on attention

Cognitive dysfunction, particularly attentional impairment, is a core feature of many psychiatric disorders, yet is inadequately addressed by current treatments. Development of targeted therapeutics for the remediation of attentional deficits requires knowledge of underlying neurocircuit, cellular, and molecular mechanisms that cannot be directly assayed in the clinic. This level of detail can only be acquired by testing animals in cross-species translatable attentional paradigms, in combination with preclinical neuroscience techniques. The 5-choice continuous performance test (5C-CPT) and rodent continuous performance test (rCPT) represent the current state of the art of preclinical assessment of the most commonly studied subtype of attention: sustained attention, or vigilance. These tasks present animals with continuous streams of target stimuli to which they must respond (attention), in addition to non-target stimuli from which they must withhold responses (behavioral inhibition). The 5C-CPT and rCPT utilize the same measures as gold-standard clinical continuous performance tests and predict clinical efficacy of known pro-attentional drugs. They also engage common brain regions across species, although efforts to definitively establish neurophysiological construct validity are ongoing. The validity of these tasks as translational vigilance assessments enables their use in characterizing the neuropathology underlying attentional deficits of animal models of psychiatric disease, and in determining therapeutic potential of drugs ahead of clinical testing. Here, we briefly review the development and validation of such tests of attentional functioning, as well as the data they have generated pertaining to inattention, disinhibition, and impulsivity in psychiatric disorders.

Measuring attention in rats with a visual signal detection task: Signal intensity vs. signal duration

Measurement of attentional performance in animal behavioral research allows us to investigate neural mechanisms underlying attentional processes and translate results to better understand human attentional function, dysfunction and drug treatments to reverse dysfunction. One useful method to measure attention in experimental animal studies is to use an operant visual signal detection paradigm, consisting of two levers and the rapid flashing of a cue lamp to signal a reward. In this study, we tested the relative sensitivity of this task when using different variants of the stimulus signal, varying brightness or duration of the light cue. To investigate roles of different neural systems underlying attentional processes, we assessed the sensitivity of attentional performance with these two different cue variations with blockade of muscarinic acetylcholine and NMDA glutamate receptors with scopolamine and MK-801 (dizocilpine). Operant signal detection was tested using a signal light that varied in intensity (0.027, 0.269, 1.22 lx) of the signal light or in a paradigm which varied the duration (0.5 s, 1 s, 2 s) of the signal light. Both methods of assessing attention showed construct validity for producing gradients of accuracy for signal detection; the dimmest cue led to less accurate responding compared to the brighter cues, and the shortest duration led to less accuracy compared to the longer durations. However, the tests differed in their sensitivity to pharmacological disruption. With the duration test, the high dose of MK-801 along with co-exposure of scopolamine and MK-801 caused a significant reduction of hit and rejection accuracy. Conversely, the intensity variation test did not show significant differences as a function of drug exposures. These data suggest that changes in signal duration, rather than signal intensity, during operant signal detection may have higher sensitivity to detecting drug effects and be a more useful technique for examining pharmacological interventions on attentional behavior and performance.

Exposure to sevoflurane anesthesia during development does not impair aspects of attention during adulthood in rats

Exposure to general anesthetic agents during development has been associated with neurotoxicity and long-term behavioral impairments in rodents and non-human primates. The phenotype of anesthetic-induced cognitive impairment has a robust learning and memory component, however less is known about other psychological domains. Data from retrospective human patient studies suggest that children undergoing multiple procedures requiring general anesthesia are at increased risk of attention deficit hyperactivity disorder. We therefore assessed whether single or repeated exposures of neonatal rats to general anesthesia caused long-term attentional impairments. Female or male Long-Evans pups were exposed to 2.5% sevoflurane for 2h on postnatal day (P) 7, or for 2h each on P7, P10 and P13. Rats were behaviorally tested in late adolescence on the sustained attention task and on the attentional set shifting task. There was no compelling evidence for anesthetic-induced impairment in attentional processing in adult rats exposed to general anesthesia as neonates. These results suggest that, at least at the developmental stage tested here, the phenotype of anesthetic-induced cognitive impairment does not involve disruptions to attentional processing.

Orexin A-induced enhancement of attentional processing in rats: role of basal forebrain neurons

RATIONALE

Orexins are neuropeptides released in multiple brain regions from neurons that originate within the lateral hypothalamus and contiguous perfornical area. The basal forebrain, a structure implicated in attentional processing, receives orexinergic inputs. Our previous work demonstrated that administration of an orexin-1 receptor antagonist, SB-334867, systemically or via infusion directly into the basal forebrain, can disrupt performance in a task that places explicit demands on attentional processing.

OBJECTIVES

Given that the orexin-1 receptor binds orexin A with high affinity, we tested whether orexin A could enhance attention in rats.

METHODS

Attentional performance was assessed using a task that required discrimination of variable duration visual signals from trials when no signal was presented. We also tested whether infusions of orexin A into the lateral ventricle could attenuate deficits following lesions of medial prefrontal cortical cholinergic projections that arise from the basal forebrain.

RESULTS

Infusions of orexin A into the basal forebrain attenuated distracter-induced decreases in attentional performance. Orexin A attenuated deficits in lesioned animals when a visual distracter was presented.

CONCLUSION

The present results support the view that orexin A can enhance attentional performance via actions in the basal forebrain and may be beneficial for some conditions characterized by attentional dysfunction due to disruption of cortical cholinergic inputs.

Developing treatments for cognitive deficits in schizophrenia: the challenge of translation

Schizophrenia is a life-long debilitating mental disorder affecting tens of millions of people worldwide. The serendipitous discovery of antipsychotics focused pharmaceutical research on developing a better antipsychotic. Our understanding of the disorder has advanced however, with the knowledge that cognitive enhancers are required for patients in order to improve their everyday lives. While antipsychotics treat psychosis, they do not enhance cognition and hence are not antischizophrenics. Developing pro-cognitive therapeutics has been extremely difficult, however, especially when no approved treatment exists. In lieu of stumbling on an efficacious treatment, developing targeted compounds can be facilitated by understanding the neural mechanisms underlying altered cognitive functioning in patients. Equally importantly, these cognitive domains will need to be measured similarly in animals and humans so that novel targets can be tested prior to conducting expensive clinical trials. To date, the limited similarity of testing across species has resulted in a translational bottleneck. In this review, we emphasize that schizophrenia is a disorder characterized by abnormal cognitive behavior. Quantifying these abnormalities using tasks having cross-species validity would enable the quantification of comparable processes in rodents. This approach would increase the likelihood that the neural substrates underlying relevant behaviors will be conserved across species. Hence, we detail cross-species tasks which can be used to test the effects of manipulations relevant to schizophrenia and putative therapeutics. Such tasks offer the hope of providing a bridge between non-clinical and clinical testing that will eventually lead to treatments developed specifically for patients with deficient cognition.

Attention

The ability to focus one's attention on important environmental stimuli while ignoring irrelevant stimuli is fundamental to human cognition and intellectual function. Attention is inextricably linked to perception, learning and memory, and executive function; however, it is often impaired in a variety of neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, depression, and attention deficit hyperactivity disorder (ADHD). Accordingly, attention is considered as an important therapeutic target in these disorders. The purpose of this chapter is to provide an overview of the most common behavioral paradigms of attention that have been used in animals (particularly rodents) and to review the literature where these tasks have been employed to elucidate neurobiological substrates of attention as well as to evaluate novel pharmacological agents for their potential as treatments for disorders of attention. These paradigms include two tasks of sustained attention that were developed as rodent analogues of the human Continuous Performance Task (CPT), the Five-Choice Serial Reaction Time Task (5-CSRTT) and the more recently introduced Five-Choice Continuous Performance Task (5C-CPT), and the Signal Detection Task (SDT) which was designed to emphasize temporal components of attention.

Subsurface cistern (SSC) proliferation in Purkinje cells of the rat cerebellum in response to acute and chronic exposure to paint thinner: A light and electron microscopy study

Intentional inhalation and occupational exposure are two ways humans are exposed to thinner, a widely employed solvent in industry. Inhalation of thinner induces toxic effects in various organs, with the cerebellum being one of the most affected structures of the CNS. The aim of this work was to describe specific structural alterations of cerebellum Purkinje cells in rats following exposure to thinner for 16 weeks. A histological analysis of the cerebellum of solvent-exposed rats revealed swollen Purkinje cell dendrites surrounded by empty space, and electronic microscopy showed an increase in the number of subsurface cisterns (SSCs) within their dendritic processes. After a period of non-exposure, the number of SSCs decreased without reaching normal levels, suggesting a degree of plasticity. Purkinje cell SSCs, which are derived from smooth endoplasmic reticulum, contain inositol trisphosphate receptors (IP3Rs), ryanodine receptors (RR), and a recently identified characteristic cluster of large conductance calcium-activated potassium (BKCa) channels. We found that SSCs in Purkinje cell dendrites were closely associated with mitochondria, and immunofluorescence microscopy showed higher levels of RR and calbindin receptors (CB), in Purkinje cells of exposed than normal rats. These changes are probably related to behavioral manifestations of cerebellar alterations, such as imbalance and ataxia, consistent with the suggested involvement of increases in SSCs in ataxia in rats and humans. This increase in SSCs, taken together with the localization of RR, IP3R and BKCa proteins in this structure, suggests altered intracellular calcium-buffering processes in the Purkinje cells of thinner-exposed rats.

Effects of a novel CB1 agonist on visual attention in male rats: role of strategy and expectancy in task accuracy

The effects of cannabinoid CB1 agonists (including Δ9-tetrahydrocannabinol, the main psychoactive component of marijuana) on attention are uncertain, with reports of impairments, no effects, and occasionally performance enhancements. To better understand these effects, we sought to uncover a role of changing online (within-session) strategy as a possible mediator of the effects of the novel, potent CB1 agonist AM 4054, on a model of sustained attention in male Sprague-Dawley rats. In this operant, two-choice reaction time (RT) task, AM 4054 decreased accuracy in an asymmetric manner; that is, performance was spared on one lever but impaired on the other. Furthermore, this pattern was enhanced by the outcome of the previous trial such that AM 4054 strengthened a win-stay strategy on the "preferred" lever and a lose-shift strategy on the "nonpreferred" lever. This pattern is often found in tests of expectancy; therefore, in a second experiment AM 4054 enhanced expectancy that we engendered by altering the probability of the two stimulus cues. Accuracy was impaired in reporting the less frequent cue, but only after two or more presentations of the more frequent cue. Taking the results of the experiments together, AM 4054 engendered expectancy by increasing the role of previous trial location and outcome on performance of future trials, diminishing stimulus control (and therefore, accuracy). This novel effect of CB1 receptor agonism may contribute to the deleterious effects of cannabinoids on attention.

Repeated visual distracter exposure enhances new discrimination learning and sustained attention task performance in rats

Repeated exposure to distraction requires attentional effort to restore task performance. However, the impact of repeated distracter exposure and exertion of attentional effort on new learning has not been examined. In the present experiment, rats were trained in a two-lever sustained attention task. Rats then continued to train, for 12 sessions, in this task either with or without a flashing houselight distracter throughout the session. The flashing houselight transiently decreased attentional performance. Trials that were part of a new light-location discrimination task were then interspersed within the sustained attention task sessions. The frequency of these new light-location discrimination trials increased with additional training. Rats exposed to the distracter exhibited higher accuracy levels during some blocks of sessions in the new light-location discrimination task trials and in the remaining sustained attention task trials compared to rats that were not exposed to the distracter. The effects of repeated distracter exposure are interpreted in the context of an "occasion-setting" model that has been used to describe performance in this task.

Task demands dissociate the effects of muscarinic M1 receptor blockade and protein kinase C inhibition on attentional performance in rats

The cholinergic system is known to be necessary for normal attentional processing. However, the receptors and mechanisms mediating the effects of acetylcholine on attention remain unclear. Previous work in our laboratory suggested that cholinergic muscarinic receptors are critical for maintaining performance in an attention-demanding task in rats. We examined the role of the muscarinic M(1) receptor and protein kinase C (PKC), which is activated by the M(1) receptor, in attention task performance. Rats were trained in an attention-demanding task requiring discrimination of brief (500, 100, 25 ms) visual signals from trials with no signal presentation. The effects of muscarinic M(1) receptor blockade were assessed by administering dicyclomine (0-5.0 mg/kg). The effects of PKC inhibition were assessed by administering chelerythrine chloride (0-2.0 mg/kg). Dicyclomine decreased the accuracy of detecting longer signals in this attention task, including when attentional demands were increased by flashing a houselight throughout the session. Chelerythrine chloride decreased the accuracy of signal detection in the standard version of the task but not when the houselight was flashed throughout the session. The present findings indicate that muscarinic M(1) receptors are critical for maintaining performance when attentional demands are increased, and that PKC activity may contribute to some aspects of attentional performance.

Posterior parietal cortex dynamically ranks topographic signals via cholinergic influence

The hypothesis to be discussed in this review is that posterior parietal cortex (PPC) is directly involved in selecting relevant stimuli and filtering irrelevant distractors. The PPC receives input from several sensory modalities and integrates them in part to direct the allocation of resources to optimize gains. In conjunction with prefrontal cortex, nucleus accumbens, and basal forebrain cholinergic nuclei, it comprises a network mediating sustained attentional performance. Numerous anatomical, neurophysiological, and lesion studies have substantiated the notion that the basic functions of the PPC are conserved from rodents to humans. One such function is the detection and selection of relevant stimuli necessary for making optimal choices or responses. The issues to be addressed here are how behaviorally relevant targets recruit oscillatory potentials and spiking activity of posterior parietal neurons compared to similar yet irrelevant stimuli. Further, the influence of cortical cholinergic input to PPC in learning and decision-making is also discussed. I propose that these neurophysiological correlates of attention are transmitted to frontal cortical areas contributing to the top-down selection of stimuli in a timely manner.

Behavioral animal models to assess pro-cognitive treatments for schizophrenia

Cognitive dysfunction is a core aspect of schizophrenia that constitutes a major obstacle toward reintegration of patients into society. Although multiple cognitive deficits are evident in schizophrenia patients, no medication is currently approved for their amelioration. Although consensus clinical test batteries have been developed for the assessment of putative cognition enhancers in patients with schizophrenia, parallel animal tests remain to be validated. Having no approved treatment for cognitive symptoms means no positive control can be used to examine pharmacological predictive validity of animal models. Thus, focus has been placed on animal paradigms that have demonstrable construct validity for the cognitive domain being assessed.This review describes the growing arsenal of animal paradigms under development that have putative construct validity to cognitive domains affected in schizophrenia. We discuss (1) the construct validity of the paradigms; (2) compounds developed to investigate putative treatment targets; and (3) manipulations used to first impair task performance. Focus is placed on the paradigm design, including how the use of multivariate assessments can provide evidence that main effects of treatment are not confounded by extraneous effects.

Attention-modulating effects of cognitive enhancers

Attention can be readily measured in experimental animal models. Animal models of attention have been used to better understand the neural systems involved in attention, how attention is impaired, and how therapeutic treatments can ameliorate attentional deficits. This review focuses on the ways in which animal models are used to better understand the neuronal mechanism of attention and how to develop new therapeutic treatments for attentional impairment. Several behavioral test methods have been developed for experimental animal studies of attention, including a 5-choice serial reaction time task (5-CSRTT), a signal detection task (SDT), and a novel object recognition (NOR) test. These tasks can be used together with genetic, lesion, pharmacological and behavioral models of attentional impairment to test the efficacy of novel therapeutic treatments. The most prominent genetic model is the spontaneously hypertensive rat (SHR). Well-characterized lesion models include frontal cortical or hippocampal lesions. Pharmacological models include challenge with the NMDA glutamate antagonist dizocilpine (MK-801), the nicotinic cholinergic antagonist mecamylamine and the muscarinic cholinergic antagonist scopolamine. Behavioral models include distracting stimuli and attenuated target stimuli. Important validation of these behavioral tests and models of attentional impairments for developing effective treatments for attentional dysfunction is the fact that stimulant treatments effective for attention deficit hyperactivity disorder (ADHD), such as methylphenidate (Ritalin®), are effective in the experimental animal models. Newer lines of treatment including nicotinic agonists, α4β2 nicotinic receptor desensitizers, and histamine H₃ antagonists, have also been found to be effective in improving attention in these animal models. Good carryover has also been seen for the attentional improvement caused by nicotine in experimental animal models and in human populations. Animal models of attention can be effectively used for the development of new treatments of attentional impairment in ADHD and other syndromes in which have attentional impairments occur, such as Alzheimer's disease and schizophrenia.

Animal models of schizophrenia

Schizophrenia may well represent one of the most heterogenous mental disorders in human history. This heterogeneity encompasses (1) etiology; where numerous putative genetic and environmental factors may contribute to disease manifestation, (2) symptomatology; with symptoms characterized by group; positive--behaviors not normally present in healthy subjects (e.g. hallucinations), negative--reduced expression of normal behaviors (e.g. reduced joy), and cognitive--reduced cognitive capabilities separable from negative symptoms (e.g. impaired attention), and (3) individual response variation to treatment. The complexity of this uniquely human disorder has complicated the development of suitable animal models with which to assay putative therapeutics. Moreover, the development of animal models is further limited by a lack of positive controls because currently approved therapeutics only addresses psychotic symptoms, with minor negative symptom treatment. Despite these complexities however, many animal models of schizophrenia have been developed mainly focusing on modeling individual symptoms. Validation criteria have been established to assay the utility of these models, determining the (1) face, (2) predictive, (3) construct, and (4) etiological validities, as well as (5) reproducibility of each model. Many of these models have been created following the development of major hypotheses of schizophrenia, including the dopaminergic, glutamatergic, and neurodevelopmental hypotheses. The former two models have largely consisted of manipulating these neurotransmitter systems to produce behavioral abnormalities with some relevance to symptoms or putative etiology of schizophrenia. Given the serotonergic link to hallucinations and cholinergic link to attention, other models have manipulated these systems also. Finally, there has also been a drive toward creating mouse models of schizophrenia utilizing transgenic technology. Thus, there are opportunities to combine both environmental and genetic factors to create more suitable models of schizophrenia. More sophisticated animal tasks are also being created with which to ascertain whether these models produce behavioral abnormalities consistent with patients with schizophrenia. While animal models of schizophrenia continue to be developed, we must be cognizant that (1) validating these models are limited to the degree by which Clinicians can provide relevant information on the behavior of these patients, and (2) any putative treatments that are developed are also likely to be given with concurrent antipsychotic treatment. While our knowledge of this devastating disorder increases and our animal models and tasks with which to measure their behaviors become more sophisticated, caution must still be taken when validating these models to limit complications when introducing putative therapeutics to human trials.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

The toxic effects of toluene on the optokinetic nystagmus in pigmented rats

The effects of 375 mgm(-3) (100 ppm) toluene in air inhalation were evaluated on pigmented rats during either repeated exposures over five consecutive days 3h a day or during a single 4-h exposure. At the end of the inhalation period, the animals were returned to fresh air to evaluate their ability to recover optokinetic performance. The optokinetic responses were analyzed using a magnetic search coil technique previously described. After repeated toluene exposure, the eye position at rest of all the rats was unsteady. In response to visual stimulation, the eye velocity was slower and more irregular than in the control state. At the end of the stimulation, the environment of the animals became stationary, but the eye did not immediately return to a fixed stable position. A similar effect was observed after a single exposure. An increase of the optokinetic deficit was observed after single or repeated 375 mgm(-3) toluene exposures. No recovery was observed even after a single exposure. In view of the fact that toluene is a widely used solvent, these results show that inhalation of low concentrations, even for short single exposures, must be taken into account, because gaze destabilization could cause vertigo symptoms.

Attentional demands for demonstrating deficits following intrabasalis infusions of 192 IgG-saporin

Previous research has shown that basal forebrain cholinergic inputs to the cerebral cortex are necessary for attentional processing. However, the key components of attention-demanding tasks for demonstrating deficits following loss of basal forebrain corticopetal cholinergic neurons are unclear. In the present experiment, rats were trained in a visual cued discrimination task with limited explicit attentional demands and then received intrabasalis infusions of the immunotoxin, 192 IgG-saporin, or saline. Postsurgically, attentional demands were increased by decreasing the signal duration or the intertrial interval or by increasing the variability of these parameters. Subsequently, rats were trained in a task that required discrimination of successively presented signals and "blank" trials with no signal presentation. Again, attentional demands were increased by manipulating signal duration or the intertrial interval. Finally, all rats were trained in a task with both the signal duration and the intertrial interval designed to increase attentional demands. Compared to sham-lesioned animals, lesioned animals exhibited deficits in signal detection only during the successive discrimination task with both the signal duration and intertrial interval shorter and variable. The present data suggest that attentional deficits following loss of cortical cholinergic inputs result from overall attentional task demands rather than being dependent on any single task parameter.

Assessment of sustained and divided attention in rats

Behavioral tasks must be evaluated in terms of the cognitive functions they require in order to be performed. All of the tasks described in this chapter can be used with each of four experimental manipulations: stimulation of a single brain region by drugs or small electrical current, impairment of normal function by production of a lesion or administration of appropriate pharmacological agents, recording of brain activity during the performance of a specific behavioral task, or behavioral phenotyping of transgenic and knockout mice for genes expressed in specific brain regions. This unit describes protocols for the radial arm maze task and the water maze task, both of which require intact spatial memory abilities.

Characterization of the effects of inhaled perchloroethylene on sustained attention in rats performing a visual signal detection task

The aliphatic hydrocarbon perchloroethylene (PCE) has been associated with neurobehavioral dysfunction including reduced attention in humans. The current study sought to assess the effects of inhaled PCE on sustained attention in rats performing a visual signal detection task (SDT). Due to its similarities in physiological effect to toluene and trichloroethylene (TCE), two other commonly used volatile organic compounds (VOCs) known to reduce attention in rats, we hypothesized (1) that acute inhalation of PCE (0, 500, 1000, 1500 ppm) would disrupt performance of the SDT in rats; (2) that impaired accuracy would result from changes in attention to the visual signal; and (3) that these acute effects would diminish upon repetition of exposure. PCE impaired performance of the sustained attention task as evidenced by reduced accuracy [P(correct): 500 to 1500 ppm], elevated response time [RT: 1000 and 1500 ppm] and reduced number of trials completed [1500 ppm]. These effects were concentration-related and either increased (RT and trial completions) or remained constant [P(correct)] across the 60-min test session. The PCE-induced reduction in accuracy was primarily due to an increase in false alarms, a pattern consistent with reduced attention to the signal. A repeat of the exposures resulted in smaller effects on these performance measures. Thus, like toluene and TCE, inhaled PCE acutely impaired sustained attention in rats, and its potency weakened upon repetition of the exposure.

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

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

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

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

Quantitative Comparisons of the Acute Neurotoxicity of Toluene in Rats and Humans

The behavioral and neurophysiological effects of acute exposure to toluene are the most thoroughly explored of all the hydrocarbon solvents. Behavioral effects have been experimentally studied in humans and other species, for example, rats. The existence of both rat and human dosimetric data offers the opportunity to quantitatively compare the relative sensitivity to acute toluene exposure. The purpose of this study was to fit dose-effect curves to existing data and to estimate the dose-equivalence equation (DEE) between rats and humans. The DEE gives the doses that produce the same magnitude of effect in the two species. Doses were brain concentrations of toluene estimated from physiologically based pharmacokinetic models. Human experiments measuring toluene effects on choice reaction time (CRT) were meta-analyzed. Rat studies employed various dependent variables: amplitude of visual-evoked potentials (VEPs), signal detection (SIGDET) accuracy (ACCU) and reaction time (RT), and escape-avoidance (ES-AV) behaviors. Comparison of dose-effect functions showed that human and rat sensitivity was practically the same for those two task regimens that exerted the least control over the behaviors being measured (VEP in rats and CRT in humans) and the sensitivity was progressively lower for SIGDET RT, SIGDET ACCU, and ES-AV behaviors in rats. These results suggested that the sensitivity to impairment by toluene depends on the strength of control over the measured behavior rather than on the species being tested. This interpretation suggests that (1) sensitivity to toluene would be equivalent in humans and rats if both species performed behaviors that were controlled to the same extent, (2) the most sensitive tests of neurobehavioral effects would be those in which least control is exerted on the behavior being measured, and (3) effects of toluene in humans may be estimated using the DEEs from rat studies despite differences in the amount of control exerted by the experimental regimen or differences in the behaviors under investigation.

Repeated inhalation of toluene by rats performing a signal detection task leads to behavioral tolerance on some performance measures

Previous work showed that trichloroethylene (TCE) impairs sustained attention as evidenced by a reduction in accuracy and elevation of response latencies in rats trained to perform a visual signal detection task (SDT). This work also showed that these effects abate during repeated exposures if rats inhale TCE while performing the SDT. The present experiment sought to determine whether toluene, another commonly-used solvent, would induce tolerance similarly if inhaled repeatedly during SDT testing. Sixteen male, Long-Evans rats were trained to perform the SDT. Upon completion of training, rats were divided into 2 groups. In Phase I, concentration-effect functions were determined for toluene (0, 1200, 1600, 2000, 2400 ppm) in both groups. Toluene reduced the proportion of correct responses [P(correct)], and increased response time (RT) and response failures. In Phase II, Group-Tol inhaled 1600 ppm toluene while Group-Air inhaled clean air during 11 daily SDT sessions. In Group-Tol the effect of toluene on P(correct) abated after 3 days, while RT remained elevated for the duration of the repeated exposures. In Phase III, toluene concentration-effect functions were re-determined for both groups. Group-Air remained impaired on all test measures, whereas for Group-Tol, toluene did not reduce P(correct), but continued to increase RT. These data confirm our previous hypothesis that animals can develop tolerance to chemical exposures that impair appetitively-motivated behaviors if that impairment leads to loss of reinforcement.

Evaluation of muscarinic and nicotinic receptor antagonists on attention and working memory

Cholinergic receptor antagonists are commonly used to model attentional and mnemonic impairments associated with neuropsychiatric disorders such as Alzheimer's disease. However, few studies have systematically assessed the effects of these drugs following manipulations that affect attention or working memory within the same task. In the present experiment, rats were trained to discriminate visual signals from "blank" trials when no signal was presented. This task was modified to include retention intervals on some trials to tax working memory. During standard task performance, rats received systemic injections of the muscarinic receptor antagonist, scopolamine, or of the nicotinic receptor antagonist, mecamylamine. A second experiment tested the effects on this task of co-administering doses of scopolamine and mecamylamine that, when administered alone, did not significantly affect task performance. Scopolamine (0.3 and 1.0 mg/kg) decreased detection of 500 ms signals but did not affect accurate identification of non-signals. Scopolamine did not differentially affect performance across the retention interval. Elevated omission rates were associated with high doses of scopolamine or mecamylamine. Combination drug treatment was associated with decreased signal detection and elevated omission rates. Collectively, the data suggest that muscarinic and nicotinic receptor antagonists do not exclusively impair working memory.

The last decade of solvent research in animal models of abuse: mechanistic and behavioral studies

The abuse of volatile organic solvents (inhalants) leads to diverse sequelae at levels ranging from the cell to the whole organism. This paper reviews findings from the last 10 years of animal models investigating the behavioral and mechanistic effects of solvent abuse. In research with animal models of inhalant abuse, NMDA, GABA(A), glycine, nicotine, and 5HT(3) receptors appear to be important targets of action for several abused solvents with emerging evidence suggesting that other receptor subtypes and nerve membrane ion channels may be involved as well. The behavioral effects vary in magnitude and duration among the solvents investigated. The behavioral effects of acute and chronic inhalant abuse include motor impairment, alterations in spontaneous motor activity, anticonvulsant effects, anxiolytic effects, sensory effects, and effects on learning, memory and operant behavior (e.g., response rates and discriminative stimulus effects). In addition, repeated exposure to these solvents may produce tolerance, dependence and/or sensitization to these effects.

Sustained attention in the mouse: A study of the relationship with the cerebellum

To explore the role of the cerebellum in sustained attention, the authors tested lurcher, wildtype, and lurcher chimeric mice with zero, normal, and variable numbers of Purkinje cells, respectively, in a previously validated task of sustained attention. Results indicate that lurcher mice had a deficit in performance likely related to their motor disability, whereas lurcher chimeras performed similarly to wildtype controls. Presentation of auditory or visual distracters caused deficits in the performance of all mice that were specific to either signal (auditory) or non-signal (visual) events. The authors' results do not support a role of the cerebellum in sustained attention, instead indicating that behavioral changes are an indirect result of motor deficits.

Neurobehavioural evaluation and kinetics of inhalation of constant or fluctuating toluene concentrations in human volunteers

The health risks of inhalation exposure to volatile organic solvents may not only depend on the total external dose, but also on the pattern of exposure. It has been suggested that exposure to regularly occurring peak concentrations may have a stronger impact on the brain than constant exposure at the same average level. Recent animal experimental studies conducted in our laboratory using relatively high concentrations of toluene have shown different effects on discrimination performance and motor activity during and after exposure, depending on the exposure scenario. Relevance of these findings for man was evaluated in a volunteer study in which 11 healthy men (age 20-49 years) were exposed by inhalation for 4h to either a constant concentration of 40ppm toluene or to three 30-min exposure peaks at 110ppm during this 4h period. Selected tests from the Neurobehavioural Evaluation System (NES) were performed repeatedly during and after exposure. Blood concentrations of toluene as well as urinary o-cresol excretion were measured at relevant time points. The results show that toluene concentration in blood increased during constant exposure and fluctuated during occupationally relevant peak exposures. Presumably, brain concentrations showed similar qualitative patterns. No clear changes were observed on neurobehavioural measures of motor performance, attention, perceptual coding and memory, or on measures of mood and affect. The exposure conditions do not seem to induce significant acute changes in central nervous system function similar to those observed at much higher concentrations in animals, although a statistical correlation was found between one motor performance test (Finger Tapping Test with alternating hands) and blood toluene concentrations. Urinary o-cresol excretion appeared to be significantly higher during the first 2h after exposure.

Postnatal ethanol exposure disrupts signal detection in adult rats

Human prenatal ethanol exposure that occurs during a period of increased synaptogenesis known as the "brain growth spurt" has been associated with significant impairments in attention, learning, and memory. The present experiment assessed whether administration of ethanol during the brain growth spurt in the rat, which occurs shortly after birth, disrupts attentional performance. Rats were administered 5.25 g/kg/day ethanol via intragastric intubation from postnatal days (PD) 4-9, sham-intubation, or no intubation (naïve). Beginning at PD 90, animals were trained to asymptotic performance in a two-lever attention task that required discrimination of brief visual signals from trials with no signal presentation. Finally, manipulations of background noise and inter-trial interval duration were conducted. Early postnatal ethanol administration did not differentially affect acquisition of the attention task. However, after rats were trained to asymptotic performance levels, those previously exposed to ethanol demonstrated a deficit in detection of signals but not of non-signals compared to sham-intubated and naïve rats. The signal detection deficit persisted whenever these animals were re-trained in the standard task, but further task manipulations failed to interact with ethanol pretreatment. The present data support the hypothesis that early postnatal ethanol administration disrupts aspects of attentional processing in the rat.

Effects of excitotoxic thalamic intralaminar nuclei lesions on attention and working memory

In rats, lesions of the thalamic intralaminar nuclei (ILn) impair measures of working memory, but it is unclear whether alterations of attention contribute to the mnemonic deficits. The present experiment tested the effects of ILn lesions on a two-lever attention task that required discrimination of visual signals and non-signals. Rats were trained presurgically in the task and then received sham surgery or infusions of n-methyl-d-aspartate (NMDA) into the ILn to induce excitotoxic lesions. ILn lesions transiently decreased accurate detection of signals. ILn lesions also increased omissions. Compared to sham-lesioned rats, ILn-lesioned animals were not differentially affected when task demands were increased by presenting a visual distracter. Finally, a retention interval was incorporated into the task to assess whether the lesions affected acquisition of a working memory version of this behavioral paradigm. Unlike sham-lesioned animals, ILn-lesioned rats did not demonstrate a significant improvement in signal detection when a retention interval was introduced. The transient lesion-induced deficits in the attention task suggest that, in rats, the ILn may contribute to aspects of attentional processing, but through neural re-organization or activity in other regions, there is compensation for the loss of ILn functioning. The ILn appear to be necessary for maintaining performance when working memory demands are increased.

Behavioural effects and kinetics in brain in response to inhalation of constant or fluctuating toluene concentrations in the rat

The toxicity of exposure by inhalation to organic solvents may not only be related to the total external dose, but also to the pattern of exposure. In this study the impact of the exposure scenario on the behavioural effects of the model solvent toluene in rats was investigated. Rats were exposed for 7.5h to either a constant concentration or fluctuating concentrations at total external dose levels of 20,000ppmh and 10,000ppmh. Different effects on measures of visual discrimination performance were observed in rats exposed to a constant or fluctuating concentrations, and when rats were tested immediately or sometime after the end of exposure to fluctuating concentrations. Motor activity was also differently affected by different exposure scenarios. Physiologically based toxicokinetic (PBTK) modelling was used to predict the toxicokinetics of toluene induced by these different exposure scenarios. The model was calibrated by measuring toluene concentrations in blood and brain during and after exposure. The results show that the acute effects of toluene on behaviour do not depend only on the concentration and duration of exposure, but primarily on the pattern of exposure.

Volatile organic compounds inhibit human and rat neuronal nicotinic acetylcholine receptors expressed in Xenopus oocytes

The relative sensitivity of rats and humans to volatile organic compounds (VOCs) such as toluene (TOL) and perchloroethylene (PERC) is unknown and adds to uncertainty in assessing risks for human exposures to VOCs. Recent studies have suggested that ion channels, including nicotinic acetylcholine receptors (nAChRs), are targets of TOL effects. However, studies comparing TOL effects on human and rat ligand-gated ion channels have not been conducted. To examine potential toxicodynamic differences between these species, the sensitivity of human and rat nAChRs to TOL was assessed. Since PERC has similar effects, in vivo, to TOL, effects of PERC on nAChR function were also examined. Two-electrode voltage-clamp techniques were utilized to measure acetylcholine-induced currents in neuronal nAChRs (alpha4beta2, alpha3beta2, and alpha7) expressed in Xenopus oocytes. PERC (0.065 mM) inhibited alpha7 nAChR currents by 60.1 +/- 4.0% (human, n = 7) and 40 +/- 3.5% (rat, n = 5), and inhibited alpha4beta2 nAChR currents by 42.0 +/- 5.2% (human, n = 6) and 52.2 +/- 5.5% (rat, n = 8). Likewise, alpha3beta2 nAChRs were significantly inhibited by 62.2 +/- 3.8% (human, n = 7) and 62.4 +/- 4.3% (rat, n = 8) in the presence of 0.065 mM PERC. TOL also inhibited both rat and human alpha7, alpha4beta2, and alpha3beta2 nAChRs. Statistical analysis indicated that although there was not a species (human vs. rat) difference with PERC (0.0015-0.065 mM) or TOL (0.03-0.9 mM) inhibition of alpha7, alpha4beta2, or alpha3beta2 nAChRs, all receptor types were more sensitive to PERC than TOL. These results demonstrate that human and rat nACh receptors represent a sensitive target for VOCs. This toxicodynamic information will help decrease the uncertainty associated with animal to human extrapolations in the risk assessment of VOCs.

Introduction of a retention interval in a sustained attention task in rats: effects of a visual distracter and increasing the inter-trial interval

The impact of manipulating explicit attentional demands on working memory has not been well studied in rodents. The present experiment was designed to test the effects of incorporating a retention interval in a two-lever sustained attention task that requires discrimination of visual signals and non-signals and that has previously been shown to yield valid measures of attention in the rat. Upon establishing baseline performance, additional manipulations, including presentation of a visual distracter and increasing the length and variability of the inter-trial interval were conducted. During baseline conditions, accurate detection of signals, but not non-signals, decreased as the retention interval was increased. Presentation of a flashing houselight throughout the session eliminated delay-dependent detection of signals. Increasing the inter-trial interval improved detection of signals and decreased detection of non-signals at the longest retention interval. Finally, increasing the variability of the inter-trial interval did not have significant effects on performance above and beyond the effects of increasing the inter-trial interval. The present experiment demonstrates that manipulation of explicit attentional demands can alter working memory performance in the rat. This task may be employed to understand the neuropharmacological and neuroanatomical substrates mediating memory while attentional load is systematically varied.

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.

Signal detection behavior in humans and rats: a comparison with matched tasks

Animal models of human cognitive processes are essential for studying the neurobiological mechanisms of these processes and for developing therapies for intoxication and neurodegenerative diseases. A discrete-trial signal detection task was developed for assessing sustained attention in rats; a previous study showed that rats perform as predicted from the human sustained attention literature. In this study, we measured the behavior of humans in a task formally homologous to the task for rats, varying two of the three parameters previously shown to affect performance in rats. Signal quality was manipulated by varying the increment in the intensity of a lamp. Trial rate was varied among values of 4, 7, and 10 trials/min. Accuracy of signal detection was quantified by the proportion of correct detections of the signal (P(hit)) and the proportion of false alarms (P(fa), i.e. incorrect responses on non-signal trials). As with rats, P(hit) in humans increased with increasing signal intensity whereas P(fa) did not. Like rats, humans were sensitive to the trial rate, though the change in behavior depended on the sex of the subject. These data show that visual signal detection behavior in rats and humans is controlled similarly by two important parameters, and suggest that this task assesses similar processes of sustained attention in the two species.

Toluene inhibits voltage-sensitive calcium channels expressed in pheochromocytoma cells

Commercial solvents such as toluene are commonly used as drugs of abuse by children and adolescents. The cellular and molecular sites and mechanisms of actions of these compounds are not well studied but their effects on behavior resemble those of central nervous system depressants such as alcohol, barbiturates and benzodiazepines. In this study, the effects of toluene on voltage-sensitive calcium channels (VSCCs) were measured in pheochromocytoma cells. The KCl-induced rise in intracellular calcium as measured by calcium imaging was almost completely blocked by the dihydropyridine calcium channel antagonist nifedipine verifying that undifferentiated pheochromocytoma cells express mainly the L-type of calcium channel. Toluene (0.3-3000 microM) by itself did not affect intracellular calcium levels in resting cells but dose-dependently inhibited the KCl-induced rise in calcium. This inhibition was substantially reversed upon washout of the toluene-containing solution. KCl-dependent increases in intracellular calcium in cells differentiated with nerve growth factor (NGF) were largely insensitive to nifedipine. Toluene produced a greater inhibition of the KCl response in NGF treated cells as compared with undifferentiated cells. A similar finding was obtained when whole-cell patch-clamp-electrophysiology was used to directly monitor the effects of toluene on voltage-activated calcium currents in undifferentiated and differentiated cells. These results show that dihydropyridine sensitive and insensitive calcium channels are inhibited by toluene and may represent important sites of action for this compound.

Characterizing tolerance to trichloroethylene (TCE): effects of repeated inhalation of TCE on performance of a signal detection task in rats

Previous work showed that rats develop tolerance to the acute behavioral effects of trichloroethylene (TCE) on signal detection if they inhale TCE while performing the task and that this tolerance depends more upon learning than upon changes in metabolism of TCE. The present study sought to characterize this tolerance by assessing signal detection in rats during three phases of TCE exposures. Tolerance was induced in Phase 1 (daily 1-h test sessions concurrent with TCE exposure), extinguished in Phase 2 (daily tests in air with intermittent probe tests in TCE), and reinduced in Phase 3. Original induction in Phase 1 required 2 weeks, whereas reinduction in Phase 3 required less than 1 week. Tolerance persisted for 2 (accuracy) or 8 weeks [response time] in Phase 2 and was resistant to changes in test conditions in Phase 3. The slow induction, gradual extinction, savings during reinduction and lack of disruption from altered test conditions suggest mediation by instrumental learning processes. These data and most other evidence for behavioral tolerance to solvents can be explained by solvent-induced loss of reinforcement.

Specificity of cognitive impairment from Pfiesteria piscicida exposure in rats: attention and visual function versus behavioral plasticity

Pfiesteria piscicida is a toxic dinoflagellate that has caused massive fish kills in estuaries along the East Coast of the United States, and exposure of humans to toxic Pfiesteria has been associated with cognitive impairment. A visual signal detection task was used to determine the possible importance of attentional and visual processes in Pfiesteria effects on cognitive function. Adult female rats were trained to perform the signal detection task. After training, the rats were injected subcutaneously with fish culture water containing toxic Pfiesteria (35,600 or 106,800 cells of Pfiesteria/kg of rat body weight) or with (control) fish culture water containing no Pfiesteria. Effects of toxic Pfiesteria on maintenance of signal detection behavior were assessed for 2 weeks after treatment. Then, the signal-response contingencies were reversed. After the discrimination was reestablished on the reversed levers, the rats received a second dose of toxic Pfiesteria. The rats were again tested for 2 weeks, after which a second reversal was imposed. Pfiesteria did not affect behavior in the signal detection task during 2 weeks of prereversal testing after either exposure. However, a significant Pfiesteria-induced deficit emerged when the signal-response contingencies were reversed. These findings suggest that Pfiesteria-induced deficits emerge during periods of behavioral transition and not during performance of previously learned tasks.

Comparing cognitive and screening tests for neurotoxicity. Effects of acute chlorpyrifos on visual signal detection and a neurobehavioral test battery in rats

It is often assumed that cognitive function is more sensitive to neurotoxic chemicals than are the unconditioned behaviors employed in neurobehavioral screens; however, direct comparisons of the sensitivity of these test methods are lacking. The present studies were conducted to compare the effects of the widely used cholinesterase-inhibiting insecticide, chlorpyrifos (O,O'-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothionate, CPF), on a visual signal detection task (SDT) with its effects on a neurobehavioral test battery. Adult male Long-Evans rats were trained to perform the SDT, dosed with CPF, and then assessed with both test instruments. Oral CPF (50 mg/kg) impaired signal detection for 8 days, and subcutaneous CPF (250 mg/kg) did so for 4 weeks. CPF (30 and 50 mg/kg po and 250 mg/kg sc) also lowered activity in the test battery for up to 18 days. Thus, CPF impaired attention and altered behavior in the test battery in the same dose ranges under two very different dosing scenarios.

Attention as a target of intoxication: insights and methods from studies of drug abuse

A symposium was convened to discuss recent developments in the assessment of attention and the effects of drugs and toxic chemicals on attention at the 17th annual meeting of the Behavioral Toxicology Society on May 1, 1999, in Research Triangle Park, NC. Speakers addressed issues including the methodology of assessing cognitive function, the neurobiology of specific aspects of attention, the dual roles of attention as a target of intoxication and as a mediating variable in the development of addiction to psychoactive drugs, the changes in attention that accompany neuropsychological disorders of schizophrenia, senile dementia of the Alzheimer type and attention deficit hyperactivity disorder, and potential therapies for these disorders. This article provides an overview of the objectives of the symposium, followed by summaries of each of the talks given.

Behavioral and electrophysiological estimates of visual thresholds in awake rats treated with 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Visual thresholds for luminance increments were obtained behaviorally and electrophysiologically from rats exposed to a polychlorinated biphenyl (PCB) during development. Male Long-Evans rats exposed to 0, 0.25, or 1.0 microg/kg/day of 3,3',4,4', 5-pentachlorobiphenyl (PCB 126) through gestation and weaning were trained as adults to perform a signal detection task. Estimates of threshold were derived from psychometric functions for each animal relating the proportion of hits to signal intensity. Thresholds derived under three luminance conditions did not differ significantly among the PCB-treated groups. After behavioral testing was completed, flash-evoked potentials were recorded from dark-adapted awake animals. Peak amplitudes increased linearly over approximately 3 log units of intensity. Extrapolations to 0 amplitude along the linear portion of the amplitude-log intensity functions produced estimates of absolute threshold of -5.44 to -5.53 log cd/m(2)-s. Waveforms recorded from awake animals had a large late negative component that was absent in previously reported anesthetized preparations. Developmental exposure to PCB 126 had no significant effect on absolute threshold or peak amplitudes and latencies.

Behavioral components of tolerance to repeated inhalation of trichloroethylene (TCE) in rats

The possibility that the acute neurotoxic effects of organic solvents change with repeated exposure will affect risk assessment of these pollutants. We observed previously that rats inhaling trichloroethylene (TCE) showed a progressive attenuation of impairment of signal detection behavior across several weeks of intermittent exposure, suggesting the development of tolerance. Here, we explored the development of tolerance to TCE during two weeks of daily exposures, and the degree to which learned behavioral modifications ("behavioral tolerance") could account for the effect. Adult Long-Evans rats were trained to perform a visual signal detection task (SDT) in which a press on one lever yielded food if a visual stimulus (a "signal") had occurred on that trial, and a press on a second lever produced food if no signal had been presented. In two experiments, with 2000 and 2400 ppm of TCE respectively, trained rats were divided into two groups (n = 8/group) with equivalent accuracy and then exposed to TCE in two-phase studies. In Phase 1, one group of rats received daily SDT tests paired with 70-min TCE exposures, followed by 70-min exposures to clean air after testing. The other group received daily SDT tests in clean air, followed by 70-min exposures to TCE (unpaired exposure and testing). All rats thus received the same number and daily sequence of exposures to TCE that differed only in the pairing with SDT testing. Both concentrations of TCE disrupted performance of the paired groups and this disruption abated over the 9 days of exposure. In Phase 2, the pairing of exposure and test conditions were reversed for the two groups. The groups that were shifted from unpaired to paired exposures (Unpaired-Paired groups) showed qualitatively similar patterns of deficit and recovery as did the rats whose tests were initially paired with TCE (Paired-Unpaired groups), indicating that task-specific learning was involved in the development of tolerance. Quantitative differences in the magnitude and duration of the effects of TCE in the two groups indicated that other factors, not specific to the SDT, also contributed to the development tolerance to TCE. Published by Elsevier Science Inc.

Toluene and temporal discrimination in rats: effects on accuracy, discriminability, and time estimation

Thirty-five rats acquired a temporal discrimination of seven signal durations in an operant chamber with two levers. If a 2-s or an 8-s tone signal was presented, rats were required to press one lever ("short" response) or the other lever ("long" response) to be reinforced, respectively. Neither response was reinforced when five intermediate signals were presented. Percentages of a long response in each of the seven signals were calculated and the psychophysical function between signal durations and long response percentages was obtained. Intraperitoneal injections of 50 mg/kg and 100 mg/kg toluene steepened a gradient of the psychophysical function and elevated correct responses in 2-s and 8-s signals. The function showed lower difference limen and Weber fraction. Accuracy and discriminability of temporal discrimination were enhanced. However, 400 mg/kg and 600 mg/kg toluene resulted in a shallower gradient and reduced correct responses. Higher difference limen and Weber fraction were also obtained. Accuracy and discriminability deteriorated and, in particular, behavioral depression was observed for 600 mg/kg toluene. The point of indifference was changed and dose-related overestimation or underestimation of time were suggested to occur. Blood toluene levels were reported to be 11-18 microg/ml for 50-100 mg/kg toluene and 47-73 microg/ml for 400-600 mg/kg toluene. It is speculated that temporal discrimination was sharpened by 10-20 microg/ml toluene in blood and disrupted by 50-70 microg/ml toluene in blood.

Behavioral assessments of learning and attention in rats exposed perinatally to 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Evidence from humans suggests that cognitive dysfunction may result from perinatal exposure to polychlorinated biphenyls (PCBs), and the results of some animal research with PCBs have been interpreted in terms of possible impairment of attention. Long-Evans rats were fed 3,3',4,4',5-pentachlorobiphenyl (PCB 126), a coplanar congener, at doses of 0.25 or 1 microgram/kg/day [corrected] throughout gestation and nursing. Male offspring of these rats were trained as adults to perform 2 tests of attention for food reward. First, a cued target-detection task, modeled after Posner's covert orienting method for humans, was used to assess visuospatial attention. In this task, a visual target stimulus was presented in 1 visual hemifield on each trial, preceded either by a valid cue, an invalid cue, or no cue. A valid cue appeared in the same hemifield as the target, and an invalid cue appeared in the opposite hemifield. As expected, valid cues increased accuracy and speed of target detection and invalid cues decreased accuracy and speed; moreover, these effects were systematically related to changes in cue intensity and target duration. However, perinatal exposure to PCB 126 did not affect acquisition or performance of this task. The second task assessed sustained attention by means of a signal detection method in which a brief, spatially-constant but temporally unpredictable, visual signal indicated which of 2 responses would yield food. Varying the intensity of the signal greatly affected the probability of correctly reporting the signal. Perinatal exposure to PCB 126 did not affect acquisition of the response rule or performance of the task. Finally, all rats were challenged with chlordiazepoxide (CDP) at doses of 0, 3, 5, 8, or 12 mg/kg SC, 20 min before testing in the sustained attention task. In control rats, low doses (3, 5, and 8 mg/kg) of CDP reduced accuracy at low signal intensities only, suggesting an increase in visual threshold. The high dose of CDP reduced accuracy at all signal intensities and increased the false-alarm rate as well, suggesting an impairment of attention. The rats exposed perinatally to PCB 126 at 0.25 micrograms/kg [corrected] were unaffected by CDP, and those exposed to PCB 126 at 1 microgram/kg [corrected] showed a smaller decrement in performance after CDP than did the controls. Taken together, these data provide little support for the possibility that perinatal exposure to PCB 126 causes deficits in attention, but suggest that PCB 126 may alter GABA-mediated pathways in the CNS during development.