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Persistence of Masking Responses to Light in Mice Lacking Rods and Cones

Overt rhythms of locomotor behavior are a result of two complementary processes. The first mechanism is the synchronization of an endogenous clock, which in turn directs the animal to be active in the day or the night; this is called entrainment. The second mechanism involves an acute response to light, which inhibits or promotes activity, depending on whether the species is nocturnal or diurnal; this is called masking (Aschoff, 1960; review in Mrosovsky, 1999). The fact that masking can occur in hamsters with lesions of the SCN (Redlin and Mrosovsky, 1999) and in mice lacking cryptochromes (van der Horst et al., 1999; Vitaterna et al., 1999), that is, in animals lacking any persistent circadian rhythm to be entrained, is further evidence that entrainment and masking are behaviorally and physiologically different responses to light. Although masking may be of considerable importance in influencing the amplitude of overt rhythms, it has received much less study than has entrainment. Indeed, the main concern of rhythms researchers has been to exclude masking as much as possible from their experiments. At the receptor level at least, however, masking and entrainment may have considerable commonalities, in that they are both spared in retinally degenerate (rd/rd) mutant mice (Foster et al., 1991; Mrosovsky, 1994). Eventually, rd/rd mice come to lose all their rods, and many cones degenerate secondarily (CarterDawson et al., 1978; Foster et al., 1991). Nevertheless, a few cones remain, and it is possible that such residual cones might be enough to mediate responses to light. This has been ruled out for phase shifting, which has been shown to persist even in mice genetically modified to lack both rods and cones (Freedman et al., 1999). The current experiment was undertaken to determine if masking was also spared in such rodless coneless mice.