Assessment of 1,2-dimethylhydrazine in bone marrow micronucleus assay: variations in protocol and response.

The effect of variations in experimental protocol on the assessment of the genotoxicity of 1,2-dimethylhydrazine (DMH) in the bone marrow micronucleus assay was determined. The incidence of micronuclei (MN) in the bone marrow of CBA mice treated with DMH (either intraperitoneally (i.p.) or orally) was found to be significantly greater than that observed in C57B1/6J mice using the same dose and dosing regimen. With i.p. injection, DMH, at doses of 20 and 50 mg/kg, was found to be positive in the bone marrow MN test in CBA mice only. In C57B1/6J mice, DMH (i.p.) was found to be positive at only the 50 mg/kg dose. With oral administration, DMH was positive in the MN test only at the 50 mg/kg dose and only in CBA mice. No significant difference in the percentage of MN was observed when 300, 500, or 1,000 polychromatic erythrocytes (PCEs) were scored following a single treatment of DMH. Cyclophosphamide (CY) was found to induce a dose-dependent increase in the percentage of MN observed in the bone marrow of C57B1/6J mice. DMH tested positive in the colon nuclear aberration (NA) assay in both strains of mice using both i.p. and oral routes of administration, although C57B1/6J mice were found to be more sensitive than CBA mice. No significant difference was observed regarding the percentage of NAs observed in the colon between mice injected i.p. or orally gavaged.

Comparison of the activity of topically applied pesticides and the herbicide 2,4-D in two short-term in vivo assays of genotoxicity in the mouse.

Genotoxicity of eight topically applied compounds was determined using the bone marrow micronucleus (MN) test and hair follicle nuclear aberration (NA) assay in CD1 mice. Twenty-four hours after a single treatment, cyclophosphamide (CY), applied at doses corresponding to 1/4, 1/8, 1/16, and 1/32 of the published dermal LD50, and N-methyl-N-nitrosourea (MNU), applied at 1/4, 1/8, and 1/16 of the published dermal LD50, were found to increase the incidence of NA in a dose-dependent manner. The frequency of MN was significantly increased only at the highest dose of CY. Using the same protocol, six pesticides applied in dimethyl sulfoxide (DMSO) at doses of 1/8, 1/16, and 1/32 of the dermal LD50 were investigated. Aminocarb and chlordane induced a dose-dependent increase in the frequency of NA, while there was an observed increase in NA incidence at only the highest doses of dichlorvos (DDVP), 4,4'-DDT (DDT), and 2,4-dichlorophenoxyacetic acid (2,4-D). No effect was observed with fenitrothion on nuclear aberrations in hair follicles. Except for the highest dose of chlordane, none of the pesticides tested positive in the bone marrow micronucleus test. Serum cholinesterase levels were reduced to 70 +/- 4.7% of the DMSO control level with DDVP, 57 +/- 8.2% with aminocarb, and 60.3 +/- 4.8% with fenitrothion, indicating some systemic activity with these topically applied agents. The data suggest that aminocarb, chlordane, DDVP, DDT, and 2,4-D are genotoxic as determined by the NA assay and that this assay may be more useful in detecting topically applied genotoxic agents than the more often used bone marrow micronucleus test.

Nongenotoxicity of minoxidil in murine hair follicles as determined by the nuclear aberration assay.

A 1-cm2 area on the back of CD1 mice was prepared for topical application of minoxidil, N-methyl-N-nitrosourea (MNU), or cyclophosphamide (CY) by clipping or plucking hair from a patch of skin. Plucking stimulates hair follicle cell division while clipping does not. Minoxidil was topically administered for 8 consecutive days. CY or MNU was administered topically once on the eighth day postplucking. The incidence of nuclear aberrations and mitotic figures were measured in hair follicles while frequency of micronuclei and the ratio of RBC/PCE were measured in the bone marrow. Results with minoxidil showed no increase in either nuclear aberrations in the hair follicle or micronuclei in the bone marrow. These results suggest that topically applied minoxidil is not genotoxic. In contrast, a dose-dependent effect of MNU on the incidence of nuclear aberrations in the hair follicle was seen. CY induced a dose-dependent increase in the incidence of micronuclei in the bone marrow and in nuclear aberrations in the hair follicle after topical application. Minoxidil applied to clipped mice significantly increased the incidence of mitotic figures above that seen in both the clipped and plucked controls. This suggests that minoxidil is a mitogenic agent in the hair follicle. These findings are consistent with the success of topically applied minoxidil in the treatment of alopecia areata.