A fundamental property of circadian rhythms is the free-running period expressed by organisms when isolated from environmental periodicity. The physiological determinants of the free-running period, including variation among and within individuals and among species, are not known. The circadian rhythms of mammals are regulated by a circadian pacemaker within the suprachiasmatic nucleus (SCN) of the hypothalamus. To examine possible determinants of the free-running period, one or two SCNs were transplanted into hamsters that had their own SCNs ablated. Wheel-running behavior was measured to estimate the free-running period of restored rhythmicity. Hosts received grafts containing either the left or right SCN from a single fetus or both SCNs from a single fetus. In some cases, both the left and right SCNs from a single fetus restored rhythmicity in different hosts, demonstrating that each of the right and left SCN alone is a competent circadian pacemaker. The average free-running period of the restored rhythms was significantly longer in hamsters that received both of the SCNs from a single fetus. The sizes of grafts were estimated using immunoreactivity for vasoactive intestinal polypeptide as a marker of SCN tissue. Grafts never grew to be larger than an intact SCN, and a graft only 6.5% the size of the combined left and right intact SCNs restored rhythmicity. The average volume of grafted SCN in hamsters that received two SCNs was larger than that in hamsters that received a single SCN. The results demonstrate that SCN graft volume and/or the number of SCNs that comprise the graft influence the free-running period.
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