Peripheral and central nervous system lesions caused by triethyl- and trimethyltin salts in rats

Behaviors in the Home Cage Reveal Toxicity: Recent Findings and Proposals for the Future

Nervous system impairment is prominent among signs of chemical toxicity in humans and animals, yet evaluation of behavioral and neurologic responses is seldom included in premarket screening. The sensitivity and validity of automatically recorded rodent locomotor activity, whether inside or outside of the home cage, justifies its inclusion in first-tier testing. Home cage behaviors are studied in the toxicologic laboratory using quantitative techniques from behavioral neuroscience. A practical, noninvasive, automated system was developed and validated at New York University, in accord with Federal guidelines for testing neurotoxicity. Effects of neurotoxicants on motor activity, eating, drinking, and the daily cycle of rest-activity indicate sensitivity to a variety of chemicals as well as new avenues to the understanding of mechanisms of toxicity. The rat's pattern of nocturnal activity is particularly sensitive to neurotoxicants and thus deserves additional attention. The coefficient of variability of various end points did not correlate with sensitivity to toxicants. This underscores the need for behavioral data to supplement theoretical considerations in test selection. The system's advantages are economy, high data capacity, humaneness, accessible and well-known end points, widely available equipment, automation, and the potential for direct comparisons of several different animal species.

A consistent, quantifiable, and graded rat lumbosacral spinal cord injury model

The purpose of this study is to develop a rat lumbosacral spinal cord injury (SCI) model that causes consistent motoneuronal loss and behavior deficits. Most SCI models focus on the thoracic or cervical spinal cord. Lumbosacral SCI accounts for about one third of human SCI but no standardized lumbosacral model is available for evaluating therapies. Twenty-six adult female Sprague-Dawley rats were randomized to three groups: sham (n=9), 25 mm (n=8), and 50 mm (n=9). Sham rats had laminectomy only, while 25 mm and 50 mm rats were injured by dropping a 10 g rod from a height of 25 mm or 50 mm, respectively, onto the L4-5 spinal cord at the T13/L1 vertebral junction. We measured footprint length (FL), toe spreading (TS), intermediate toe spreading (ITS), and sciatic function index (SFI) from walking footprints, and static toe spreading (STS), static intermediate toe spreading (SITS), and static sciatic index (SSI) from standing footprints. At six weeks, we assessed neuronal and white matter loss, quantified axons, diameter, and myelin thickness in the peroneal and tibial nerves, and measured cross-sectional areas of tibialis anterior and gastrocnemius muscle fibers. The result shows that peroneal and tibial motoneurons were respectively distributed in 4.71 mm and 5.01 mm columns in the spinal cord. Dropping a 10-g weight from 25 mm or 50 mm caused 1.5 mm or 3.75 mm gaps in peroneal and tibial motoneuronal columns, respectively, and increased spinal cord white matter loss. Fifty millimeter contusions significantly increased FL and reduced TS, ITS, STS, SITS, SFI, and SSI more than 25 mm contusions, and resulted in smaller axon and myelinated axon diameters in tibial and peroneal nerves and greater atrophy of gastrocnemius and anterior tibialis muscles, than 25 mm contusions. This model of lumbosacral SCI produces consistent and graded loss of white matter, motoneuronal loss, peripheral nerve axonal changes, and anterior tibialis and gastrocnemius muscles atrophy in rats.

Evaluation of developmental neurotoxicity of organotins via drinking water in rats: Monomethyl tin

Organotins such as monomethyltin (MMT) are widely used as heat stabilizers in PVC and CPVC piping, which results in their presence in drinking water supplies. Concern for neurotoxicity produced by organotin exposure during development has been raised by published findings of a deficit on a runway learning task in rat pups perinatally exposed to MMT (Noland EA, Taylor DH, Bull RJ. Monomethyl and trimethyltin compounds induce learning deficiencies in young rats. Neurobehav Toxicol Teratol 1982;4:539-44). The objective of these studies was to replicate the earlier publication and further define the dose-response characteristics of MMT following perinatal exposure. In Experiment 1, female Sprague-Dawley rats were exposed via drinking water to MMT (0, 10, 50, 245 ppm) before mating and throughout gestation and lactation (until weaning at postnatal day [PND] 21). Behavioral assessments of the offspring included: a runway test (PND 11) in which the rat pups learned to negotiate a runway for dry suckling reward; motor activity habituation (PNDs 13, 17, and 21); learning in the Morris water maze (as adults). Other endpoints in the offspring included measures of apoptosis (DNA fragmentation) at PND 22 and as adults, as well as brain weights and neuropathological evaluation at PND 2, 12, 22, and as adults. There were no effects on any measure of growth, development, cognitive function, or apoptosis following MMT exposure. There was a trend towards decreased brain weight in the high dose group. In addition, there was vacuolation of the neuropil in a focal area of the cerebral cortex of the adult offspring in all MMT dose groups (1-3 rats per treatment group). In Experiment 2, pregnant rats were exposed from gestational day 6 until weaning to 500 ppm MMT in drinking water. The offspring behavioral assessments again included the runway task (PND 11), motor activity habituation (PND 17), and Morris water maze (as adults). In this second study, MMT-exposed females consumed significantly less water than the controls throughout both gestation and lactation, although neither dam nor pup weights were affected. As in Experiment 1, MMT-exposure did not alter pup runway performance, motor activity, or cognitive function. These results indicate that perinatal exposure to MMT, even at concentrations which decrease fluid intake, does not result in significant neurobehavioral or cognitive deficits. While mild neuropathological lesions were observed in the adult offspring, the biological significance of this restricted finding is unclear.

The potential for vigabatrin-induced intramyelinic edema in humans

PURPOSE

Vigabatrin (Sabril, Hoechst Marion Roussel) is an antiepilepsy drug (AED) presently marketed in 64 countries for the treatment of partial and secondarily generalized seizures. Vigabatrin (VGB) is marketed in a subset of these countries for the treatment of infantile spasms. Clinical experience in humans has shown that VGB provides effective seizure control with a wide margin of safety. However, animal toxicity studies raised concern when prolonged administration of VGB was shown to induce intramyelinic edema (IME) in some laboratory animal species.

METHODS

Animal and human data were reviewed with respect to the potential for VGB-induced IME. Surveillance of patients receiving VGB in clinical trials or by prescription has been conducted for >15 years to identify patients developing clinical abnormalities that might be IME related.

RESULTS

The histologic lesions of VGB-induced IME in animals are reliably reproduced and correlate with changes in multimodality evoked potentials (EPs) and magnetic resonance imaging (MRI). Numerous studies of the effects of VGB on EP and MRI in epilepsy patients have demonstrated no clear-cut IME-related changes in these modalities. Additionally, autopsy and surgical brain samples from VGB-treated patients have been scrutinized for potential IME histopathology. In an estimated 350,000 patient-years of VGB exposure (approximately 175,000 patients exposed for 2 years at an average dose of 2 g/day), no definite case of VGB-induced IME has been identified.

CONCLUSIONS

Comprehensive review of a variety of sources of data failed to identify any definite case of IME in humans treated with VGB.

Triethylbenzene-induced sensorimotor neuropathy in rats

1,2,4-Triethylbenzene (1,2,4-TEB) and 1,3,5-triethylbenzene (1,3,5-TEB) were administered orally to male Sprague-Dawley rats. Experimental and appropriate control rats were examined electrophysiologically for motor and sensory conduction velocities (MCV and SCV), and the amplitudes of the sensory (ASAP) and muscular action potentials (AMAP), at bi-weekly intervals. Oral administration of 1,2,4-TEB (200 or 400 mg kg-1, once daily, 4 days per week for 8 weeks) produced a time- and dose-dependent decrease in MCV, SCV, AMAP and ASAP. Rats treated with 1,2,4-TEB exhibited a bluish discoloration of the skin and the urine was greyish-greenish. No changes in MCV, SCV, AMAP and ASAP developed in rats given 1,3,5-TEB orally (200 or 400 mg kg-1, daily, 4 days per week for 8 weeks). The results indicate that 1,2,4-TEB is a neurotoxic isomer of TEB and that the presence of two ethyl radicals in the ortho-position on an aromatic ring could be a critical molecular arrangement resulting in chromogenic and neurotoxic properties.

Correlation between behavioral and pathological changes in the evaluation of neurotoxicity

Recently the EPA has implemented new guidelines under Section 4 of TSCA to undertake neurotoxicity testing. It is expected that other agencies, both national and international, will follow suit. The evaluation of neurotoxicity will be based primarily on behavioral and morphological observations and particularly on the correlation between them. Initially, considerable information will have to be gathered from various laboratories to form the background data on which decisions of toxic effects can be made. This paper presents our experience to date with behavioral and neuropathological procedures for the evaluation of chemical compounds or dietary regimens in rats for potential neurotoxicity. Reference is made to acrylamide, 2,5-hexanedione, 3,3'-iminodipropionitrile, amphetamine, physostigmine, ethanol, triethyltin, food and water deprivation, and carbon tetrachloride. The question of whether every change induced by xenobiotics can be considered as a sign of toxicity is discussed.