We have already shown that forestomach lesions induced by 24 weeks administration of genotoxic carcinogens, 8-nitroquinoline (8-NQ), 2-(2-furyl)-3-(5-nitro-2-furyl)acrylamide (AF-2), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or N-methylnitrosourethane (MNUR), do not regress within 24 weeks of cessation of the chemical exposure. In contrast, simple or papillary hyperplasias induced by 24 weeks administration of non-genotoxic carcinogens, butylated hydroxyanisole (BHA), caffeic acid (CA), sesamol or 4-methoxyphenol (4-MP), largely disappear. In the present study, to examine the mechanisms of the reversibility of proliferative forestomach lesions, representative non-genotoxic carcinogens, BHA, CA or 4-MP, and one genotoxic carcinogen, MNUR, were given to male F344 rats for 24 weeks, then their forestomachs were sequentially analyzed after withdrawal of the chemical insult. MNUR-induced proliferative lesions such as hyperplasias, papillomas and squamous cell carcinomas did not regress and the expression level of cell cycle regulators, cyclin D1 and p27 were not altered after cessation of the chemical insult. On the other hand, BHA-, CA- or 4-MP-induced hyperplasias clearly regressed, in association with reduction of cyclin D1 expression. During this process, expressions of p27 and cleaved caspase 3 were not altered. These findings suggest that regression of proliferative lesions induced by non-genotoxic rat forestomach carcinogens is controlled partially by a positive cell cycle regulator, cyclin D1, whereas dysregulation of this mechanism is a characteristic of the irreversible lesions induced by a genotoxic carcinogen.

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The aim of this study was to investigate the effects of repeated administrations of relatively low doses of 7,12-dimethylbenz[a]anthracene (DMBA) on the induction of mammary carcinomas (MCs) and dysplasias (MDs) in rats. DMBA at 0, 0.1, 0.2, 0.5 and 1 mg for female rats, and at 0, 0.5, 1 and 2 mg for male rats was administered twice weekly after weaning. In female rats, repeated administration of 1 mg DMBA induced MCs while 0.1 mg DMBA induced MDs in all animals. The incidence of MCs increased and the latent period shortened while the incidence of MDs decreased in a dose-dependent manner. In male rats, repeated administration of 2 mg DMBA induced MCs in all animals. The incidence of MCs increased and the latent period shortened in a dose-dependent manner while the incidence of MDs increased by repeated administration of 1 mg DMBA. These results suggest that the total exposure of chemical carcinogens may be responsible for the induction of MCs or MDs, and that MDs may be more dependent on female sex hormones than MCs.

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To investigate the association between streptozotocin (STZ)-induced painful diabetic neuropathy and the antihyperalgesic effect of capsaicin cream in rats, we first examined the antihyperalgesic effect of capsaicin cream and subsequently performed peripheral neurohistochemical examinations of neuropathic rats. Mechanical hyperalgesia occurred 2 weeks after STZ injection and persisted for 8 weeks of testing. The neuropathy was alleviated by a single application of 0.1% capsaicin cream, but not by cream base. The hyperalgesia did not decrease when the capsaicin cream was applied to normal animals. On the other hand, for dorsal root ganglion (DRG) neurons and hind paw cutaneous nerves, the neurohistochemical examination showed no difference between STZ rats and control rats with capsaicin (vanilloid) receptor subtype 1 (VR1) and NF200 double immunohistochemical staining of DRG neurons, but an increase in A-fibers was seen in the hind paw cutaneous nerves of the STZ rats compared to the control rats. Neither STZ rats nor control rats were toxically affected by the single application of 0.1% capsaicin cream. These results suggest that a single application of capsaicin cream exerts an antihyperalgesic effect in rats with painful neuropathy and increases peripheral A-fibers. However, we could not clarify how VR1 was involved in the effect in rats with STZ painful diabetic neuropathy.

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We encountered one pituitary carcinoma derived from the pars intermedia in an untreated female B6C3F1 mouse in a carcinogenicity study. A white nodule (6 × 5 × 3 mm) was observed in the base of the brain at necropsy. Tumor cells were histologically arranged in a solid sheet or nest-like structures, and had round or oval nuclei and pale eosinophilic granular cytoplasm. Sporadic mitotic figures were seen in the tumor while necrosis was a prominent feature. In addition, distinct infiltration into the brain parenchyma was observed. Immunohistochemically, tumor cells were positive for α-melanocyte stimulating hormone (α-MSH), γ-melanocyte stimulating hormone (γ-MSH), β-endorphin and adenocorticotropic hormone (ACTH), but were negative for prolactin. However, there were no histological abnormalities such as adrenocortical hyperplasia related to the pars intermedia tumor. These results suggest that the tumor cells might not have actively secreted proopiomelanocortin (POMC)-derived peptides. The present report provides additional histopathological evidence of pituitary carcinoma of the pars intermedia in aging mice.

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A pulmonary mass was observed in a 16-year-old beagle dog. In the histopathology, a well-circumscribed mass composed of cuboidal to low or tall columnar neoplastic cells proliferating in a papillary, acinar or solid pattern and abundant cholesterin deposition was observed. The tumor cells had eosinophilic to abundant clear or foamy cytoplasms. The cells were arranged around blood vessels with the nuclei at the periphery of the cells in a pseudo-rosette formation, especially in the tall columnar cells. In the immunohistochemistry, the neoplastic cells were positive for cytokeratin and surfactant apoprotein A, but not for vimentin and chromogranin A. In the electron microscopy, numerous vacuoles ranging from 0.5 to 2 μm in diameter were observed in the basal part of the neoplastic cells and some of these vacuoles contained myelin-like materials. These findings indicate that these neoplastic cells were derived from alveolar type II cells. Therefore, the pulmonary mass was diagnosed as bronchioloalveolar carcinoma.

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A lymphoma was found in a Japanese killifish (medaka), Oryzias latipes. The tumor mass was detected in the area adjacent to the left operculum. Lymphoid cells were observed in the dermis, gill, skeletal muscle, thyroid gland, kidney and thymic region. The thymus was considered the primary site because the tumor mass was located mainly at the thymic region, and tumor cells showed massive proliferation in the thymic region.

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An immunohistochemical staining method for prohibitin, a 30-kDa protein located in the inner membrane of mitochondria, is used for detection of mitochondria on formalin-fixed paraffin-embedded sections. Methapyrilene hydrochloride (MP), which is known to increase mitochondria in periportal hepatocytes, was administered once by oral gavage to male rats at a dose of 100 mg/kg. Hypertrophy with eosinophilic granules was noted in the periportal hepatocytes treated with MP. Immunohistochemical staining with prohibitin antibody demonstrated a positive reaction to eosinophilic granules. Electron microscopy showed an increased number of mitochondria in the periportal hepatocytes treated with MP compared to the control. Although electron microscopy is a useful tool to detect ultrastructural changes, an immunohistochemical examination with anti-prohibitin antibody is a simple and easy method for detecting cytoplasmic mitochondria quantitatively.

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