MUTAGENESIS AND BEHAVIORAL SCREENING FOR ALTERED CIRCADIAN ACTIVITY IDENTIFIES THE MOUSE MUTANT, WHEELS

The molecular processes underlying the generation of circadian behavio r in mammals are virtually unknown. To identify genes that regulate or alter circadian activity rhythms, a mouse mutagenesis program was ini tiated in conjunction with behavioral screening for alterations in cir cadian period (tau), a fundamental property of the biological clock. M ale mice of the inbred BALB/c strain, treated with the potent mutagen N-ethyl-N-nitrosourea were mated with wild-type hybrids. Wheel-running activity of approximate to 300 male progeny was monitored for 6-10 we eks under constant dark (DD) conditions. The tau(DD) of a single mouse (#187) was longer than the population mean by more than three standar d deviations (24.20 vs. 23.32 +/- 0.02 h; mean +/- S.E.M.; n = 277). I n addition, mouse #187 exhibited other abnormal phenotypes, including hyperactive bi-directional circling/spinning activity and an abnormal response to light. Heterozygous progeny of the founder mouse, generate d from outcrossings with wild-type C57BL/6J mice, displayed lengthened tau(DD) although approximate to 20% of the animals showed no wheel-ru nning activity despite being quite active. Under light:dark conditions , all animals displaying circling behavior that ran in the activity wh eels exhibited robust wheel-running activity at lights-ON and these an imals also showed enhanced wheel-running activity in constant light co nditions. The genetic dissection of the complex behavior associated wi th this mutation was facilitated by the previously described genetic m apping of the mutant locus causing circling behavior, designated Wheel s (Whl), to the subcentromeric portion of mouse chromosome 4. In this report, the same locus is shown to be responsible for the abnormal res ponses to light and presumably for the altered circadian behavior. Cha racterization of the gene altered in the novel Whl mutation will contr ibute to understanding the molecular elements involved in mammalian ci rcadian regulation.