MUTANT CIRCADIAN PERIOD AS A MARKER OF SUPRACHIASMATIC NUCLEUS FUNCTION

Fetal suprachiasmatic nucleus (SCN) tissue transplanted into the third ventricle of hamsters bearing complete SCN lesions restores the circa dian locomotor rhythm with a period that depends exclusively on the ge netically determined period of the tissue donor. If the host is only p artially lesioned and thus retains rhythmicity with its own geneticall y determined period, an implant from an animal of a different genotype can induce a second rhythm with a period determined by the donor geno type. Both rhythms can be present simultaneously in the record of such a ''temporal chimera,'' interacting only superficially (i.e., not at the level of the pacemaker). Our data support the interpretation that under such circumstances the graft is able to capture part of the loco motor output of the circadian system, but does not make functional con nections with the host SCN pacemaking system.