Similarities of toluene and o-cresol neuroexcitation in rats

The Future of Neurotoxicology: A Neuroelectrophysiological Viewpoint

Neuroelectrophysiology is an old science, dating to the 18th century when electrical activity in nerves was discovered. Such discoveries have led to a variety of neurophysiological techniques, ranging from basic neuroscience to clinical applications. These clinical applications allow assessment of complex neurological functions such as (but not limited to) sensory perception (vision, hearing, somatosensory function), and muscle function. The ability to use similar techniques in both humans and animal models increases the ability to perform mechanistic research to investigate neurological problems. Good animal to human homology of many neurophysiological systems facilitates interpretation of data to provide cause-effect linkages to epidemiological findings. Mechanistic cellular research to screen for toxicity often includes gaps between cellular and whole animal/person neurophysiological changes, preventing understanding of the complete function of the nervous system. Building Adverse Outcome Pathways (AOPs) will allow us to begin to identify brain regions, timelines, neurotransmitters, etc. that may be Key Events (KE) in the Adverse Outcomes (AO). This requires an integrated strategy, from in vitro to in vivo (and hypothesis generation, testing, revision). Scientists need to determine intermediate levels of nervous system organization that are related to an AO and work both upstream and downstream using mechanistic approaches. Possibly more than any other organ, the brain will require networks of pathways/AOPs to allow sufficient predictive accuracy. Advancements in neurobiological techniques should be incorporated into these AOP-base neurotoxicological assessments, including interactions between many regions of the brain simultaneously. Coupled with advancements in optogenetic manipulation, complex functions of the nervous system (such as acquisition, attention, sensory perception, etc.) can be examined in real time. The integration of neurophysiological changes with changes in gene/protein expression can begin to provide the mechanistic underpinnings for biological changes. Establishment of linkages between changes in cellular physiology and those at the level of the AO will allow construction of biological pathways (AOPs) and allow development of higher throughput assays to test for changes to critical physiological circuits. To allow mechanistic/predictive toxicology of the nervous system to be protective of human populations, neuroelectrophysiology has a critical role in our future.

Effect of toluene and cresols on Na+,K+-ATPase, and serotonin in rat brain

The objective of the present trial was to evaluate the effect of toluene and o-cresol, m-cresol, and p-cresol on serotonin (5-HT), its precursor 5-hydroxytryptophane (5-HTP), Na(+),K(+)-ATPase, total ATPase, and lipid peroxidation (TBARS) in rat brain. Evaluation of lipid peroxidation was realized by means of TBARS, determination of biogenic amines and enzymes assay was carried out in brain homogenate samples using HPLC and spectrophotometry, respectively. Five groups of male Wistar rats (200 g) were treated as follow: control, toluene, o-cresol, m-cresol, and p-cresol groups, which were administered 35 mg/kgi.p. of each compound, the control group was given only glycerine as vehicle. 5-HT and 5-HTP levels increased significantly (p < 0.001) in toluene and o-cresol groups. Lipid peroxidation increased significantly (p < 0.002) in all groups. A significant increase (p < 0.001) of Na(+),K(+)-ATPase was noted in the toluene and o-cresol groups, while this enzyme was reduced in the p-cresol group compared to the control group. Total ATPase showed significant differences in the p-cresol group, compared to the control group. Based in our results, it can be concluded that toluene and all cresols groups may increase lipid peroxidation and consequently induce changes in membrane fluidity.

Toluene exposure increases aminophylline-induced seizure susceptibility in mice

The effects of toluene on the sensitivity to seizures induced by aminophylline were investigated. Mice were pretreated with an ip injection of corn oil or toluene (100-500 mg/kg) followed by a timed intravenous infusion of aminophylline at various time intervals to assess the seizure thresholds and lethal doses. Toluene increased seizure susceptibility to aminophylline in a dose- and time-dependent manner. Toluene-induced enhancement of seizure susceptibility to aminophylline occurred as early as 30 min and persisted for at least 3 days after a single administration of toluene (500 mg/kg). Treatment of benzaldehyde, one of toluene's metabolites, also showed an increase in the susceptibility to aminophylline. The enhancing effect was also observed in caffeine-induced seizures 1 h, but not 1 day after toluene treatment. These results suggest that individuals with toluene exposure may increase the risk for convulsive and even lethal complications associated with the therapeutic use of aminophylline.

Toluene inhibits synaptic transmission without causing gross morphological disturbances

The effects of toluene on synaptic transmission and neuronal morphology were investigated using guinea pig hippocampal slices. Population spikes (PS) were elicited in granule cell layer by stimulation of the perforant path and antidromic potentials (AP) were evoked in the same locii by stimulation of mossy fibers. Toluene at a concentration of 1000 micrograms/ml completely depressed post-synaptic responses within 15 min but increased the AP to 140% of its original value. The PS recovered completely when washout was begun 10 and 20 min after onset of depression, but exhibited only partial recovery following a 40 min depression. There were no evident changes in staining of axons or cell bodies after toluene treatment. These results indicate that toluene at high concentrations (1000 micrograms/ml) inhibits synaptic transmission selectively and with longer exposures causes lasting physiological effects unaccompanied by gross morphological changes.

Neurotoxicologic examination of rats exposed to 1,1,1-trichloroethane vapor for 13 weeks

Large evoked potential and EEG changes occurred in a pilot study in Fisher 344 rats during exposure to 2000 ppm of 1,1,1-trichloroethane (1,1,1-T; a cleaning solvent with anesthetic properties). In the main study, rats were evaluated for persistent nervous system effects the week following exposure to 0, 200, 630, or 2000 ppm 1,1,1-T for 6 h/day, 5 days/week, for 13 weeks. Rats were clinically examined regularly and were given a functional observational battery monthly (FOB, including forelimb and hindlimb grip performance testing). After 13 weeks of exposure, the rats were evaluated by FOB and by visual, auditory, somatosensory, and caudal nerve-evoked potentials. After functional testing, a subgroup of rats had histopathologic examination of brain, spinal cord, peripheral nerves, and limb muscles. There were no post-exposure treatment-related findings in any parameter (FOB observations plus 39 dependent variables) except for a slightly smaller forelimb grip performance in the 2000-ppm exposure group. There was no recognized toxicologic significance for the difference in forelimb grip performance; a lack of findings in any other clinical, evoked potential or morphologic parameter did not support a diagnosis of neurotoxicity.

Excitatory and inhibitory effects of toluene on neural activity in guinea pig hippocampal slices

To investigate the effect of toluene and its derivatives on neural activity, postsynaptic field potential (population spike, PS) of granule cells as well as antidromic potential (AP) and presynaptic fiber potential (FP) (perforant path) were recorded in the guinea pig hippocampal slices. Toluene at the concentration of 0.2 ng/ml to 20 micrograms/ml in the perfusion medium increased the amplitude of PS to 109-150%. Toluene also increased the amplitude of FP and AP, although the most remarkable enhancement was observed in the PS. However, toluene at the concentrations over 1000 micrograms/ml completely depressed the PS, whereas it increased the amplitude of AP to 130% of the original level. These results indicate that toluene has excitatory and inhibitory biphasic effects on neurotransmission in the hippocampal slices according to concentration applied.

The use of experimental studies to reveal suspected neurotoxic chemicals as occupational hazards: acute and chronic exposures to organic solvents

The nervous system differs from many other body organs by its central control of vital functions and its low regeneration capacity. Organic solvents have, as a group, been suspected to have neurotoxic effects. Because of their similar physical properties and the fact that in industrial uses, they are often present in various mixtures, organic solvents have also been regarded, unfortunately, to induce common neurotoxic effects. However, it is evident from experimental studies using specified exposure conditions that different organic solvents have very diverse neurotoxic effects and also that the toxic mechanism may differ between acute and chronic exposure. No specific method used to describe a neurotoxic effect or single toxic response can be used for the overall occupational risk assessment of all organic solvents. Each solvent has to be considered as having its own unique toxic effects.

Neurotoxicologic evaluation of rats after 13 weeks of inhalation exposure to dichloromethane or carbon monoxide

Male and female Fischer 344 rats were exposed to dichloromethane (methylene chloride, DCM) or carbon monoxide (CO) for 6 hr/day, 5 days/week, for 13 weeks. Since oxidative metabolism of DCM to CO and CO2 is a saturable process, DCM exposure concentrations were selected clearly below saturation (50 ppm), just below saturation (200 ppm), and well above saturation (2000 ppm). At saturation of metabolism, metabolic CO causes about 10% carboxyhemoglobinemia (COHb). Therefore, as a control for CO effects, a separate group of rats was exposed to 135 ppm CO to induce approximately 10% COHb. Postexposure functional tests included an observational battery, hindlimb grip strength, and a battery of evoked potentials (flash, auditory brainstem, somatosensory, caudal nerve). After functional tests were completed, rats from all groups were perfused with fixative and a comprehensive set of nervous tissues from the high DCM exposure group and from controls were examined by light microscopy. Although some miscellaneous functional and morphologic variations were recorded, none were related to treatment. Thus, subchronic exposures as high as 2000 ppm DCM or 135 ppm CO had no deleterious effects on any of the measures of this study.

Evoked potential changes from 13 weeks of simulated toluene abuse in rats

Fischer 344 rats were exposed to 8000 ppm toluene vapor in an 'abuse' paradigm for 13 weeks to develop an animal model for 'solvent neurotoxicity.' Exposures to toluene were multiple and short (15 to 35 min), adjusted according to tolerance. Although body weight was reduced 23% from controls, the toluene-exposed rats appeared healthy. Evoked potentials taken postexposure were, however, mildly to severely affected. Flash-evoked potentials were slow and topographically disorganized; 10 kHz tone-pip auditory brainstem responses (ABRs) had severe loss of power and loss of detail. Click and 30 kHz ABRs, somatosensory-evoked potentials, and caudal nerve action potentials were less affected. No neuropathologic changes were detected by light microscopy (perfusion fixation, special stains). Thus, postexposure multimodal functional effects were readily detected after subchronic, severe episodic exposures to toluene.

Acute effects of inhaled dichloromethane on the EEG and sensory-evoked potentials of Fischer-344 rats

Acute effects of inhaled dichloromethane on the spontaneous electroencephalogram (EEG) and sensory-evoked potentials (EPs) were characterized and compared to previously observed effects of toluene; both solvents are common components of abused solvent mixtures. Twelve adult male Fischer-344 rats with chronic epidural electrode implants served as subjects. Each rat was exposed for 60 min to 5,000, 10,000, and 15,000 ppm dichloromethane while held in a plastic restrainer that also served as a head-only exposure chamber. The sequence of exposures was counterbalanced across rats, and the exposures were separated by about one week. To characterize the time course of any changes, somatosensory and flash EPs were recorded every 5 min during the first 45 min of the exposures. As was the case with toluene, electrophysiologic waveforms recorded from different sensory systems, and components of these waveforms, reacted in different ways to dichloromethane. With respect to the FEP and SEP the two solvents produced quite different effects. Toluene increased the amplitudes of early FEP components, eliminated late components, induced oscillations in visual cortex, and had no discernible effects on component latencies. In contrast, dichloromethane eliminated the N1 component, at moderate exposure had little or no effects on amplitudes of the later components (N3 through N4), did not induce oscillations, and had significant effects on latencies. Whereas toluene dramatically increased SEP component amplitudes at moderate concentrations with diminishing effect at higher concentrations and exposure times, dichloromethane rather uniformly decreased SEP amplitude in a simple concentration-related way. Toluene and dichloromethane had similar effects on BAER component latencies. They both caused component (P1 through P5) latencies and the P1-P5 interwave time to increase. However, whereas toluene increased early and late (but not middle) component amplitudes, dichloromethane decreased the amplitudes of early and late components and increased the amplitudes of middle components. These results emphasize the acute pharmacologic specificity of different solvents and suggest that differences in chronic neurotoxicity might also be found; they also suggest that predictable interactions might be found with acute and chronic exposure to mixtures that contain such solvents.

Acute electrophysiologic effects of inhaled toluene on adult male Long-Evans rats

Experiments were carried out in Long-Evans rats to verify and extend previous findings about the effects of toluene on sensory-evoked potentials (EPs) of Fischer-344 rats. Inhalation exposures to 3000 and 8000 ppm in Long-Evans rats confirmed that toluene 1) transiently enhances certain components of somatosensory, flash- and click-evoked (brainstem) potentials, 2) increases the latencies and interwave times of brainstem auditory-evoked responses, 3) depresses late components of the flash EP, 4) induces high frequency oscillations in the visual cortex, and 5) produces both facilitatory and suppressant effects on EPs, dependent on exposure concentration and time. New results indicated that toluene 1) has similar effects on Long-Evans as it does on Fischer-344 rats, 2) increases EEG theta activity, 3) has minor effects on cortical auditory and pattern-reversal EPs (PREP), but suppresses the steady-state PREP, and 4) induces oscillations in the visual cortex, irrespective of the presence of flashes.

Multimodal effects of acute exposure to toluene evidenced by sensory-evoked potentials from Fischer-344 rats

Male Fischer-344 rats were exposed by inhalation to 500, 2000, 5000, 8000 and 16000 ppm toluene for 30 min in two experiments. Exposures up to 8000 ppm in Experiment 1 caused concentration-related changes in the click-elicited brainstem auditory-evoked response (CBAER), flash-evoked potential (FEP) and somatosensory-evoked potential (SEP). Latencies of CBAER components were prolonged and amplitudes of late components were increased by toluene. Toluene did not detectably alter the latencies of FEP or SEP components. Early FEP component-amplitudes were increased and late component-amplitudes were decreased; toluene also induced a poststimulus oscillation in the FEP. Most component-amplitudes of the SEP were substantially increased, but N2P2 amplitude appeared to be more sensitive than other components to depressant effects of the solvent. The same effects on the CBAER were observed in Experiment 2, but a more substantial increase in the amplitudes of late components elicited by tone pips suggested that frequency-dependent cochlear irritation might underlie previously observed subchronic ototoxicity. These effects were increased by exposure to 16000 ppm toluene. Effects like those observed in Experiment 1 were noted on the FEP, but the oscillations were less with exposure to 16000 than 8000 ppm. Changes in the SEP were evident within 2 minutes of exposure onset, and amplitudes increased over the course of about 15 min, leveling off or decreasing thereafter. The amplitude of the N2P2 component was again less influenced than other components during exposure to 8000 ppm and was reduced to less than baseline amplitude by 16000 ppm. Effects of concentration and rates of development and recovery were systematically related to SEP component latency. Toluene appears to have both enhancing and inhibiting effects on neural pathways serving sensory systems, depending on the modality and the site of generation of the components within modalities. A particular balance between these properties might relate to the hedonic characteristics of this abused solvent.