The tail suspension test (TST) and forced swimming test (FST) are widely used for assessing antidepressant activity and depression-like behavior. We found that CS mice show negligible immobility in inescapable situations. Quantitative trait locus (QTL) mapping using CS and C57BL/6J mice revealed significant QTLs on chromosomes 4 (FST) and 5 (TST and FST). To identify the quantitative trait gene on chromosome 5, we narrowed the QTL interval to 0.5 Mb using several congenic and subcongenic strains. Ubiquitin-specific peptidase 46 (Usp46) with a lysine codon deletion was located in this region. This deletion affected nest building, muscimol-induced righting reflex and anti-immobility effects of imipramine. The muscimol-induced current in the hippocampal CA1 pyramidal neurons and hippocampal expression of the 67-kDa isoform of glutamic acid decarboxylase were significantly decreased in the Usp46 mutant mice compared to control mice. These phenotypes were rescued in transgenic mice with bacterial artificial chromosomes containing wild-type Usp46. Thus, Usp46 affects the immobility in the TST and FST, and it is implicated in the regulation of GABA action.

The purpose of this study was to clarify the effect of decreased food consumption on evaluation of myelotoxicity in routine general toxicity studies. Male rats were divided into the following 7 groups: 12, 15, and 18 mg/kg 5-fluorouracil (5-FU) treatment groups (FU12, FU15 and FU18); dietary restriction groups (R12, R15 and R18 receiving the same amount of food as the rats in the FU12, FU15 and FU18 groups, respectively); and a nontreated control group (NT). We compared the changes in body weight, hematology and the results of cytological analyses of bone marrow and histopathology among the groups after administration and recovery periods of 14 and 7 days, respectively. At the end of the administration period, the FU15 and FU18 groups showed decreases in many hematologic and bone marrow parameters that were all similar to those in the corresponding dietary restriction groups (R15 and R18). A granulocyte abnormality (polyploidy: frequency of 1% or less) was also observed in all 5-FU treated groups. At the end of the recovery period, increases in the reticulocyte and platelet counts and extramedullary hematopoiesis of the spleen were observed in the 5-FU treated groups. These results indicate that the results of general toxicity studies in rats should be evaluated in consideration of dietary restriction effects when food consumption is decreased at about 30-40% or more. Careful morphological observation of hemocytes would be helpful in distinguishing the effect of a drug from that of dietary restriction in relation to hematological and bone marrow parameters. Performance of a recovery test to determine the reactive response of hematopoiesis is also recommended.

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ABSTRACT. We attempted to characterize the influence of undernutrition on erythropoiesis in toxicity studies. Male rats were divided into the following 5 groups: dietary restriction groups in which feeding was restricted by 33% or 66% for 14 days (R33 and R66); phlebotomy groups in which 1% or 4% of total blood volume was removed by serial phlebotomy for 14 days (PB01 and PB04); and a nontreated group (NT). Toxicological parameters such as hematology and blood chemistry were evaluated. The body weight gains in the R33 and phlebotomy groups (PB01 and PB04) were similar and were less than that observed in the NT group. Decreases in peripheral blood reticulocytes, bone marrow erythroids and the unsaturated iron binding capacity (UIBC) were observed as changes that suppressed erythropoiesis in the R33 and R66 groups. However, increases in reticulocytes and UIBC were observed as opposite changes in the phlebotomy groups. In addition, an increase in the blood urea nitrogen level and a decrease in the serum alkaline phosphatase level were observed as changes reflecting poor nutrition in the phlebotomy groups. Decreased reticulocytes which are related to poor nutrition were not observed. However, increases in those cells as reflected by a loss of blood were observed in the phlebotomy groups. Even if undernutrition suppresses erythropoiesis, the ability of erythropoiesis to respond to a demand appears to be retained. In repeated dose toxicity studies, decreased food consumption is often observed in the drug administration groups. Our study results provide useful information for hematological evaluations in toxicity studies.

The aim of this study was to prove our hypothesis that adult rats with lowering of body weight gain, rats at 12 weeks of age as an example, are suitable for evaluation of myelotoxicity. Age-related differences between young rats (6-week-old study) and adult rats (12-week-old study) were analyzed in hematological examination values. The data of the young rats were reprinted from our previous report (Miyata et al., 2009) since our hypothesis was verified by comparison with that previous report. Several experimental groups were defined for the 12-week-old study as well as for the 6-week-old study; these included 5-fluorouracil (5-FU) treated groups receiving 12, 15 and 18 mg/kg/day (FU12, FU15 and FU18), pair-feeding groups (R12, R15 and R18 receiving the same amount of food as in the FU12, FU15 and FU18 groups, respectively) and a nontreated control group. Numerous hematologic and bone marrow parameters in the 5-FU treated groups were comparable to those in the corresponding pair-feeding groups in both age studies. Generally, the influences of undernutrition were more apparent in the young rats than in the adult rats. Histopathological examinations showed a decrease in hematopoiesis in the bone marrow in the 5-FU treated and pair-feeding groups. No apparent differences were observed in the decreased hematopoiesis between the 5-FU treated and pair-feeding groups in the 6-week-old study, but a difference between these groups was noted in the 12-week-old study; decreased hematopoiesis was more frequently noted in the 5-FU treated groups. These facts suggest that adult rats are more suitable than young rats for evaluation of 5-FU-induced myelotoxicity.