Comparative histopathology of the estrous or menstrual cycle in laboratory animals.

Accurate analysis of female reproductive toxicity requires a thorough understanding the differences in and specifics of estrous or menstrual cycles between laboratory animals. There are some species differences such as the time of sex maturation, the length of the estrous or menstrual cycle, the length of the luteal phase, the number of dominant follicles or corpora lutea, the size of follicles, processes of luteinization, and hormonal changes during the estrous or menstrual cycle. Rodents have a short estrous cycle, and their ovarian cycling features are the same in both ovaries, which contain a large number of follicles and corpora lutea. The dog estrous cycle is much longer than those of other laboratory animals, and it includes a long anestrus phase. The duration of the menstrual cycle of monkeys is roughly 30 days, and their ovarian cycling features are different between the left and right ovaries. In both rodents and dogs, the theca cells invade the early luteum, mixing with granulosa cells during luteinization. However in monkeys, the theca layer dose not mix with the granulosa cells as it invaginates only slightly into the early luteum. In addition, we found that high progesterone levels after ovulation are sustained for a much shorter duration in rodents than in dogs and monkeys due to the comparatively rapid passage of the rodent luteal phase. Based on these species differences, animal species for use in ovarian toxicology studies need to be selected appropriately.

Absence of in vivo genotoxic potential and tumor initiation activity of kojic acid in the rat thyroid.

To clarify the in vivo genotoxic potential of kojic acid (KA), formation of DNA adducts and 8-hydroxy-deoxyguanosine (8-OHdG) in the thyroids of male rats subjected to dietary administration of 2% KA for 2 weeks were assessed by 32P-postlabeling analysis and with a high-performance liquid chromatography system coupled to an electrochemical detector (ECD), respectively. In addition, to investigate possible tumor initiation activity, male F344 rats were given diet containing 0, 0.02, 0.2 or 2% kojic acid for 8 weeks followed by administration of 0.1% sulfadimethoxine (SDM), a thyroid tumor promoter, in the drinking water for 23 weeks with a subsequent 13-week recovery period (two-stage thyroid tumorigenesis model). Rats given four times by s.c. injection of N-bis(2-hydroxypropyl)nitrosamine (DHPN; 700 mg/kg bw) during the initiation period followed by administration of 0.1% SDM and rats given diet containing 2% KA for the initial 8 weeks or for the entire 31 weeks of the experiment, or basal diet alone were provided as controls. DNA adducts were not formed, and the 8-OHdG level was not increased in the thyroids of rats given 2% KA for 2 weeks. In the two-stage thyroid tumorigenesis model, neither adenomas nor carcinomas were induced in the groups given 0, 0.02, 0.2 or 2% KA followed by 0.1% SDM administration, and incidences and multiplicities of focal follicular cell hyperplasias did not demonstrate any significant intergroup differences at the end of administration and recovery periods. In contrast, incidences and multiplicities of focal follicular cell hyperplasias, adenomas and carcinomas were all significantly increased in the DHPN + 0.1% SDM group. Although the incidences and multiplicities of focal follicular cell hyperplasias in the group given 2% KA for 31 weeks were greater than those in the 2% KA + 0.1% SDM group and an adenoma was observed in a rat at the end of the recovery period, no development of carcinomas was evident at either time point. No thyroid proliferative lesions were induced in the group given 2% KA for the initial 8 weeks only. The results of the present studies indicate that KA has neither in vivo genotoxic potential nor tumor initiation activity in the thyroid, and strongly suggest that the earlier observed thyroid tumorigenic activity of KA is attributable to a non-genotoxic mechanism.

Improvement of a two-stage carcinogenesis model to detect modifying effects of endocrine disrupting chemicals on thyroid carcinogenesis in rats.

In order to improve the sensitivity of our previously established thyroid carcinogenesis model and to clarify whether endocrine disrupting chemicals with weak estrogenic activity have any modifying effects on the development of thyroid proliferative lesions, 6-week-old female castrated F344 rats were first given a single subcutaneous injection of 2000 mg/kg body weight of N-bis(2-hydroxypropyl)nitrosamine. From 1 week later, they received diets with: no supplement (basal diet (BD) group); cholesterol pellets containing 0.5 mg 17 beta-estradiol 3-benzoate (EB); or diet admixed with 1000 ppm methoxychlor (MXC) or 10,000 ppm bisphenol A (BPA) for 20 weeks. Furthermore, additional groups were administered 200 ppm sulfadimethoxine (SDM) in the drinking water simultaneously with the BD, EB, MXC or BPA treatments. Thyroid follicular cell hyperplasias, adenomas and/or carcinomas were induced only in the EB+SDM group, the incidences of non-malignant lesions being significantly increased, as compared with the BD+SDM group values. Furthermore, the serum level of thyroid stimulating hormone (TSH) was significantly increased in this group. No significant variation in quantitative values for thyroid proliferative lesions or TSH levels were observed in the other treated groups. The results of the present study convincingly indicate that EB, with strong estrogenic activity, but not MXC and BPA, with weak estrogenic activities, exerts promoting effects on thyroid carcinogenesis in rats. The present modified rat two-stage thyroid carcinogenesis model appears to have advantages over our previous model for screening purposes.

Hepatocellular tumor induction in heterozygous p53-deficient CBA mice by a 26-week dietary administration of kojic acid.

In order to evaluate the tumorigenic potential of kojic acid (KA), used as a food additive for preventing enzymatic browning of crustaceans and a cosmetic agent for the purpose of skin whitening, heterozygous p53-deficient CBA [p53(+/-)] mice, which are recognized as useful for detecting genotoxic carcinogens, and wild-type littermates [p53(+/+) mice] were fed diet containing 0, 1.5, and 3% KA for 26 weeks. KA induced diffuse hypertrophy and hyperplasia of thyroid follicular epithelial cells with decreased serum thyroxine levels in both p53 (+/-) and p53 (+/+) mice, but caused no thyroid tumors. In the liver, the incidence of altered hepatocellular foci was significantly increased at 1.5 and 3% in p 53(+/-) and 1.5% in p53 (+/+) mice, and that of hepatocellular adenomas was increased at 1.5 and 3% in p 53(+/-) and 3% in p53 (+/+) mice. p53 (+/-) mice thus appeared to be more susceptible in terms of the tumorigenic dose of KA with a greater prevalence of hepatic proliferative lesions. The results of the present study indicate tumorigenic potential of KA in the liver, but not thyroid follicular epithelial cells in CBA mice and a contribution of genotoxicity on hepatocellular tumor development cannot be ruled out.

Dose-threshold for thyroid tumor-promoting effects of xylazine in rats.

To clarify the threshold dose of thyroid tumor-promoting effects of xylazine hydrochloride (XZ), male F344 rats received pulverized basal diet containing 0, 250, 500, or 1000 ppm XZ for 26 weeks with or without initiation of 2400 mg/kg N-bis(2-hydroxypropyl)nitrosamine (DHPN). Thyroid weights significantly increased in the groups with or without DHPN initiation that were given 500 ppm XZ or more. The serum thyroxine (T4) and triiodothyronine (T3) levels decreased significantly in the XZ 250 and XZ 1000 ppm groups, respectively, although there were no remarkable changes in the serum thyroid-stimulating hormone (TSH) levels. Histopathologically, follicular cell hyperplasias and adenomas were induced in the DHPN-alone and DHPN+XZ groups, and the incidences and multiplicities of these lesions in the DHPN groups treated with 500 ppm XZ or more were significantly higher than those in the DHPN alone group. These results suggest that the threshold dose of rat thyroid tumor-promoting effects of XZ is between 250 and 500 ppm under the present experimental condition.

Spatial focusing of neuronal responses induced by asynchronous two-tone stimuli in the guinea pig auditory cortex.

Spatiotemporal patterns of neuronal responses to asynchronous two-tone stimuli in the anterior field of the auditory cortex of anesthetized guinea pigs were studied using an optical recording method (12 x 12 photodiode array, voltage sensitive dye RH795). Interactions between the onset response to the first tone (masker; 5, 8, 10, 12 and 15 kHz, 200 ms) and to the second tone (probe; 10 kHz, 30 ms) with onset delays relative to the masker onset (0, 5, 10, 15 and 20 ms) were investigated. In general, two-tone interaction was suppressive rather than facilitative. At 0-10 ms probe delays, two-tone responses induced in the probe isofrequency area on the cortex tended to fuse with the masker response. At 15-20 ms probe delays, the probe response was apparently reduced, but was spatially focused and separated from the masker response. This spatial focusing of the probe response may have been due to neuronal inhibition originating after the masker onset response. These results are in agreement with psychoacoustical observations in human subjects, such as auditory segregation, and indicate that the spatial focusing of the cortical response provides a neuronal basis for detecting slightly asynchronous auditory inputs.