Influence of dosing volume on the neurotoxicity of bifenthrin.

Pyrethroids are pesticides with high insecticidal activity and relatively low potency in mammals. The influence of dosing volume on the neurobehavioral syndrome following oral acute exposure to the Type-I pyrethroid insecticide bifenthrin in corn oil was evaluated in adult male Long Evans rats. We tested bifenthrin effects at 1 and 5 ml/kg, two commonly used dose volumes in toxicological studies. Two testing times (4 and 7 h) were used in motor activity and functional observational battery (FOB) assessments. Four to eight doses were examined at either dosing condition (up to 20 or 26 mg/kg, at 1 and 5 ml/kg, respectively). Acute oral bifenthrin exposure produced toxic signs typical of Type I pyrethroids, with dose-related increases in fine tremor, decreased motor activity and grip strength, and increased pawing, head shaking, click response, and body temperature. Bifenthrin effects on motor activity and pyrethroid-specific clinical signs were approximately 2-fold more potent at 1 ml/kg than 5 ml/kg. This difference was clearly evident at 4 h and slightly attenuated at 7 h post-dosing. Benchmark dose (BMD) modeling estimated similar 2-fold potency differences in motor activity and pyrethroid-specific FOB data. These findings demonstrate that dose volume, in studies using corn oil as the vehicle influences bifenthrin potency. Further, these data suggest that inconsistent estimates of pyrethroid potency between laboratories are at least partially due to differences in dosing volume.

Effects of developmental hypothyroidism on auditory and motor function in the rat.

Deafness is a common result of severe hypothyroidism during development in humans and laboratory animals; however, little is known regarding the sensitivity of the auditory system to more moderate changes in thyroid hormone homeostasis. The current investigation compared the relative sensitivity of auditory function, motor function, and growth to the effects of moderate to severe perinatal hypothyroidism in the rat. Rats received propylthiouracil (PTU) in drinking water at concentrations of 0, 1, 5, and 25 ppm from Gestation Day 18 until postnatal day (PND) 21, and the effects on their offspring were evaluated. At 1 ppm, PTU did not affect any of the measured endpoints. Serum thyroxin concentrations were sharply reduced in the 5 and 25 ppm PTU groups at all ages sampled (PND 1, 7, 14, and 21). Marked reductions in serum triiodothyronine (T3) concentrations were also detected for all ages > or = 7 at 25 ppm PTU, whereas no effects of 5 ppm PTU on serum T3 were apparent until PND 21. Compared to the controls, pups exposed to the highest dose of PTU demonstrated a delay in eye opening, reduced body weights, decreased and/or delayed preweaning motor activity, and persistent, postweaning hyperactivity. Only slight and transient effects on eye opening and ontogeny of motor activity were seen at the intermediate dose of PTU (5 ppm). Reflex modification audiometry revealed that, compared to controls, adult offspring from the 5 and 25 ppm treatment groups showed dose-dependent auditory threshold deficits (35 to > 50 dB) at all frequencies tested (1, 4, 16, 32, and 40 kHz). Such dose-dependent effects indicate that the developing auditory system may be sensitive to mild hypothyroidism, suggesting the possible need for routine audiometric screening for infants and children at risk for iodine deficiency, myxedema, and/or exposure to thyrotoxic environmental agents.

Developmental exposure to polychlorinated biphenyls (Aroclor 1254) reduces circulating thyroid hormone concentrations and causes hearing deficits in rats.

Developmental hypothyroidism causes growth deficits, motor dysfunction, and hearing disorders in humans and animals. Therefore, environmental toxicants, such as polychlorinated biphenyls (PCBs), may secondarily affect these endpoints via thyrotoxicity. In this study, Long-Evans rats were given Aroclor 1254 (po), at 0, 1, 4, or 8 mg/kg from Gestation Day 6 through Postnatal Day (PND) 21. We evaluated the offspring at various age intervals for circulating thyroid hormone concentrations [thyroid-stimulating hormone, and free and total triiodothyronine (T3) and thyroxin (T4)], body weight, eye opening, survival, motor activity development, auditory startle response, and auditory thresholds. Circulating T4 concentrations were sharply reduced in a dose-dependent fashion in PCB-exposed groups at PND 1, 7, 14, 21, and 30 but recovered to control levels by PND 45. Moderate reductions in T3 concentrations were apparent in the 4 and 8 mg/kg groups on PND 21 and 30. Deficits in body weight gain and early eye opening were apparent in the treated pups; by weaning, pup mortality was 20% in the 4 mg/kg group and 50% at the highest dose. Motor activity was also transiently reduced in 15 day old offspring from the 8 mg/kg group. At this dose, animals showed reduced auditory startle amplitudes at PND 24, but not when tested as adults. Importantly, Aroclor 1254 caused permanent auditory deficits (20-30 dB threshold shift) at the lowest frequency tested (1 kHz) in both the 4 and 8 mg/kg groups, whereas auditory thresholds were not significantly affected at higher frequencies (4, 16, 32, or 40 kHz). These data indicate that while some effects of Aroclor 1254 exposure are dissimilar to drug-induced hypothyroidism (e.g., age of eye opening), effects on hormone levels and body weight are comparable. Detection of auditory deficits in PCB-treated animals is a novel finding and may reflect the effects of thyroid hormone disruption on the development of the cochlea.

The ototoxicity of 3,3'-iminodipropionitrile: functional and morphological evidence of cochlear damage.

Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.

Solvent-induced ototoxicity in rats: an atypical selective mid-frequency hearing deficit.

Most previous reports of ototoxicity following exposure to several volatile organic solvents have restricted testing to the low- and mid-frequencies (2-20 kHz) of the hearing range in the rat (0.25-80 kHz). We report here that inhalation exposure to styrene, mixed xylene, toluene, and 1,1,2-trichloroethylene resulted in hearing dysfunction only in the mid-frequency range and spared function at lower and higher frequencies. Adult male Long Evans rats were exposed via inhalation (whole body) in flow-through chambers. The following exposures were used: styrene, 1600 ppm; 1,1,2-trichloroethylene, 3500 ppm; toluene, 2500 ppm; mixed xylenes, 1800 ppm (N = 7-8 per group, 8 h/day for 5 days), and n-butanol, 4000 ppm (N = 10/group, 6 h/day for 5 days). Testing of auditory function was conducted 5 to 8 weeks after exposure using reflex modification audiometry (RMA). RMA thresholds were determined for frequencies from 0.5 to 40 kHz. Results indicated increased RMA thresholds for the mid-frequency tones (e.g., 8 and 16 kHz), but not higher or lower tones, for all solvents except n-butanol. Toluene and xylene also increased thresholds at 24 kHz. These data indicate that for those solvents reported thus far to cause hearing loss, the deficit is restricted to mid-frequencies in rats.

Age-dependent differences in the susceptibility of rats to deltamethrin.

Separate groups of weanling and adult rats were exposed to both behaviorally active and lethal doses of deltamethrin to examine age-dependent toxicity of a pyrethroid over a wide dose range. The acoustic startle response (ASR) was selected for comparison at low doses since it is a sensitive, quantifiable biological indicator of pyrethroid effects in rats. Acute mortality was included for comparison at the upper limit of the dose-response. Deltamethrin was administered by gavage as a single dose in corn oil for all tests. Effects on the ASR were comparable in 21- and 72-day-old rats, with a 4-mg/kg dose decreasing ASR amplitude by approximately 50% (ED50) at both ages. By comparison LD50 values in 11-, 21- and 72-day old male rats were 5.1, 11, and 81 mg/kg, respectively. Thus, 11- and 21-day-old male rats were 16 and 7 times, respectively, more sensitive than adults to acute lethality. The concentration of deltamethrin was measured in whole-brain tissue from weanling and adult males treated with ED50 and LD50 doses. The brain concentration of deltamethrin at the ED50 dose of 4 mg/kg was higher in weanling rats than adults. This suggests a possible functional difference, with weanling rats being less susceptible than adults to a low dose. By comparison, there was an equivalent concentration of deltamethrin in brain tissue following an LD50 dose of 12 mg/kg in weanling rats and 80 mg/kg in adults. These results support age-related differences in pharmacokinetics as the basis for the markedly greater sensitivity of young rats to a lethal dose of deltamethrin.

The sensitivity to 3,3'-iminodipropionitrile differs for high- and midfrequency hearing loss in the developing rat.

3,3'-Iminodipropionitrile (IDPN) has been demonstrated to produce a loss of hearing following both neonatal and adult exposures. Adult exposure induces a full spectrum hearing loss, whereas early postnatal exposure produces a high-frequency loss only. The purpose of this work was to delineate the period of development during which the rat becomes sensitive to the full ototoxic effects of IDPN. Primiparous Long Evans rats or their offspring were exposed to either saline or 300 mg/kg IDPN for three consecutive days. Ages of exposure were as follows: gestational days 15-17 or postnatal days (PND) 1-3, 5-7, 15-17, 20-22, 25-27, 30-32, 40-42, or 70-72. All animals were tested as adults for auditory thresholds to 5- and 40-kHz tones using reflex modification audiometry. Results demonstrate that adult-like susceptibility to IDPN was not reached until approximately PND 30-32. Early exposures (PND 5-22) to IDPN will induce a highfrequency selective hearing loss, sparing the lower frequency. Prenatal or early neonatal (PND 1-3) IDPN exposure resulted in a high degree of mortality (> 70%). The long period of time between the susceptible period for the high frequency (PND 5-7) and the lower frequency (PND 30-32) does not correspond to the basal to apical ontogenic profile of any one physiological or anatomical process. These data suggest either a unique site of action for IDPN in the cochlea or the possibility of two different mechanisms, one operating at early postnatal ages and one at later ages.

Auditory deficits and motor dysfunction following iminodipropionitrile administration in the rat.

The behavioral effects of 3,3'-iminodipropionitrile (IDPN) were studied using reflex modification of the acoustic startle response and figure-eight maze activity. A number of experiments were conducted with separate groups of adult male Long-Evans hooded rats exposed to saline or 50-500 mg/kg IDPN for 3 consecutive days. Auditory thresholds (reflex modification), motor activity, and grip strength were measured 1 day, and 1, 3, and 9 weeks postdosing. Reflex inhibition was monitored daily, prior to, during, and for 7 days following exposure. Auditory thresholds for 5- and 40-kHz tones were elevated approximately 25 dB and 50 dB, respectively. The onset of this auditory dysfunction in the 200-mg/kg/day group, as demonstrated by a loss of reflex inhibition, was 2 days for the 40-kHz tone and 4 days for the 5-kHz tone. Motor activity was increased up to 400% in the 200-mg/kg group, whereas there was no alteration in hindlimb grip strength. These data demonstrate dosage- and time-dependent alterations in auditory and motor function following IDPN exposure.

The effects of type I and II pyrethroids on motor activity and the acoustic startle response in the rat.

Recent data have demonstrated that the in vivo effects of low dosages of two pyrethroids, cismethrin and deltamethrin, can be differentiated. Two behavioral tests, locomotor activity and the acoustic startle response (ASR), were utilized to separate the behavioral actions of Type I and II pyrethroids using permethrin, RU11679, cypermethrin, RU26607, fenvalerate, cyfluthrin, flucythrinate, fluvalinate and p,p'-DDT. Dosage-effect functions for all compounds were determined for both figure-eight-maze activity and the ASR in the rat. All compounds were administered po in 1 ml/kg corn oil 1.5-3 hr prior to testing. All compounds produced dosage-dependent decreases in locomotor activity. The Type I compounds, permethrin and RU11679, along with p,p'-DDT, increased amplitude and had no effect on latency to onset of the ASR. In contrast, the Type II pyrethroids, cypermethrin, cyfluthrin, and flucythrinate, decreased amplitude and increased the latency to onset of the ASR. Fenvalerate increased the amplitude, had no effect on latency, but unlike the other compounds tested, increased ASR sensitization. Fluvalinate had no effect on any measure of the ASR. These data provide further evidence of the differences between the in vivo effects of low dosages of Type I and II pyrethroids, and extend the findings of our previous work to other representatives of the two classes of pyrethroids.