Implementation of a primary screen for developmental neurotoxicity.

A battery of tests to evaluate physical growth/development and neurobehavioral function was conducted with 78 litters of control Sprague-Dawley rats given purified water by intubation. The objectives of this study were to optimize test methods and to document the range and variability of the experimental endpoints. Data are presented for maternal evaluations (body weight gain, food consumption), gestation length, litter size, and postnatal survival. Pup body weight was used to assess postnatal somatic growth rate from birth to 85 days of age, while whole and regional brain weight measurements at 7, 28, and 85 days provided a more specific measure of physical growth relevant to a neurobehavioral study. Physical landmarks of development evaluated were pinna unfolding, incisor eruption, and eye opening while reflex landmarks of development evaluated were the negative geotaxis and pupillary reflexes. The mean percentage of litters acquiring a physical trait or reflex increased sigmoidally with age, and the data suggest that the potential to detect developmental delays would be optimal when ca. 90% of control litters reach the test landmark. Functional evaluations were arranged according to four testing subsets so that each litter was evaluated in each test (1 pup/sex/litter), but repeated testing on pups was minimized. Auditory and tactile startle reflexes, as well as prepulse inhibition, were measured at ages 22 days and 60-64 days and found to increase with age. A passive avoidance paradigm (age 40-43 days) was used to assess exploratory behavior (approach) and memory (avoidance). Swimming performance in a water maze was used to evaluate learning. In this test, escape times and error rates improved to their highest level by five or six trials and showed acceptable degrees of variability. Spontaneous motor activity was monitored for 23 hr at age 54-61 days to evaluate exploratory activity, photoperiod entrainment, and catecholamine-induced locomotion (amphetamine challenge). Finally, landmarks of sexual maturation (balanopreputial separation evident at 45 days of age, vaginal perforation evident at 33 days) and estrous cyclicity (4.8 cycles per 21 days) were evaluated as measures of reproductive neuroendocrine function. In sum, the test battery provided an efficient yet comprehensive screen for evaluating effects on physical growth/development and neurobehavioral function which meets practical criteria for preclinical testing of pharmaceutical agents.

Relative cerebellar weight: a potential indicator of developmental neurotoxicity.

Overt malformations do not always accompany behavioral changes resulting from perinatal exposure to certain drugs. However, the central nervous system (CNS) is the anatomical substrate for behavior, and functional defects may be accompanied by more subtle, structural alterations of the brain. The purpose of this study was to determine if changes in the weights of certain brain regions occur in rats exposed prenatally and/or perinatally to propylthiouracil (PTU) which retards functional development of the brain. Pregnant rats were dosed with PTU during gestation and/or lactation, and on postnatal day 28, auditory startle responses were measured to determine if PTU altered functional development. The brains of all pups were then dissected into 10 separate regions, dried and individually weighed. Brain weights were expressed in absolute and relative (to total brain weight) terms. Pups that were exposed to PTU from days 10 to 21 of gestation grew normally, but their startle responses to auditory stimuli were significantly different (p less than 0.05) from controls. Thus, for the purpose of this experiment, PTU acted as a developmental neurotoxicant because it altered performance on a test of neuromuscular function without being overtly teratogenic. The weights of most brain regions in PTU-treated rats were statistically comparable to controls. However, relative cerebellar weight was significantly (p less than 0.05) different. Therefore, these data suggested that relative cerebellar weight might be used to predict functional defects that appear during development following prenatal or perinatal exposure to certain neurotoxicants. Although cerebellar weight and auditory startle responses were altered by PTU, this study does not establish a causal relationship between the anatomical and functional changes that occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Design of a primary screen for developmental neurotoxicity.

A battery of tests was devised for routine use as a primary screen for developmental neurotoxicity. The battery was divided into preweaning and postweaning tests using rats as subjects. Since the rat CNS is structurally incomplete at birth, preweaning tests were predominantly physical, using specific landmarks of somatic maturation and regional brain growth as indices of normal development. Pupillary responses to light and positional responses to gravity (negative geotaxis) were also included in the preweaning battery to monitor reflex behavior, a relatively simple CNS function. The postweaning battery predominantly contained functional tests to evaluate higher order behaviors that develop after completion of neurogenesis. The postweaning tests were divided into 4 subsets designed to evaluate (I) neuromuscular function, (II) memory, (III) problem solving, and (IV) neuroendocrine function, respectively. Curiosity, rhythmicity, patency of monoamine neurons, and physical measures of brain growth were included within the subsets so as to evaluate a spectrum of CNS functions. Preliminary findings suggest that tests and instrumentation selected for the proposed battery provide an informative, objective, comprehensive and cost-efficient means to screen for developmental neurotoxicity.

Lack of in vivo mutagenicity and testicular toxicity of triamterene in mice.

Triamterene (2,4,7-triamino-6-phenylpteridine), a widely used diuretic/antihypertensive agent with weak antifolate activity, has been found to be positive in several in vitro assays for mutagenicity. The present studies were undertaken to characterize the potential mutagenic and antifolate activity of triamterene in the bone marrow and testes of mice with in vivo treatment. Triamterene had no clastogenic effects on the bone marrow at 6, 16, or 24 hr after a single oral dose of 25, 125, or 250 mg/kg. No alterations in hematopoietic cell maturation characteristic of antifolate action were observed in a dose-range study in which triamterene was orally administered to mice at 5-300 mg/kg/day for 5 days. Triamterene had no adverse effects on mating or fertility and did not induce dominant lethal mutations in the germ cells of male mice when given for 5 days at 5-100 mg/kg/day. Oral exposure to mice under identical conditions had no effect on testicular weight, DNA content, or activity of the de novo pathway for thymidine synthesis from deoxy [6-3H]uridine. The present findings are consistent with an absence of mutagenic effect and antifolate action on the bone marrow and testes with in vivo administration.

Effect of benzodiazepines on age of vaginal perforation and first estrus in mice.

Female mice which received chlordiazepoxide, diazepam, oxazepam, prazepam, flurazepam, or nitrazepam prenatally and postnatally had delays in the age of vaginal perforation and first estrus concomitant with reduced postnatal growth. Females exposed prenatally showed no growth deficits, but in four treatments had delayed vaginal opening. However, the age of first estrus was generally less than the control.

Benzodiazepines and reproduction of swiss-webster mice.

Chronic dietary administration of 6 different benzodiazepine tranquilizers (chlordiazepoxide, diazepam, oxazepam, prazepam, flurazepam, and nitrazepam) to breeding pairs of Swiss-Webster mice resulted in alterations of the normal patterns of reproductive behavior and fetal growth. Significant decreases in mating performance were seen among mice given diets containing 0.15% chlordiazepoxide, 0.05% diazepam, 0.05 and 0.15% oxazepam, 0.02 and 0.10% prazepam, 0.10% flurazepam, and 0.025% nitrazepam. Offspring in all drug treatments showed significantly depressed body weights at birth.

Benzodiazepine-induced suppression of estrous cycles in C57BL/6J mice.

Female C57BL/6J mice fed diets containing diazepam, chlordiazepoxide, oxazepam, prazepam, flurazepam, or nitrazepam exhibited significant decreases in the frequency of vaginal estrus.