Nonphotic entrainment of circadian activity rhythms in suprachiasmatic nuclei-ablated hamsters.

This study examined the role of the suprachiasmatic nuclei (SCN) in nonphotic entrainment. The wheel-running activity of SCN-ablated hamsters was recorded in constant dark (DD) and then under prolonged schedules of 2-hr daily cage changes, restricted food availability, and daily light-dark (LD) cycles. Some hamsters with very large lesions subsuming the SCN and surrounding areas exhibited significant, albeit unstable, circadian activity rhythms in DD. Some hamsters with similar ablations also showed entrained rhythms to daily cage change schedules. All hamsters showed robust rhythms entrained to a daily feeding schedule, but no hamsters showed entrainment to LD cycles. Competent circadian oscillators evidently exist outside the SCN, at least 0.5 mm or more away, and at least some are nonphotically entrainable. Weaker entrainment in animals with larger lesions suggests that nonphotically entrainable oscillators also exist within the SCN or its immediate vicinity.