FUNCTIONAL-ANALYSIS OF CIRCADIAN PACEMAKERS IN NOCTURNAL RODENTS .1. STABILITY AND LABILITY OF SPONTANEOUS FREQUENCY

The circadian pacemakers controlling activity rhythms in 4 spp. of rodents [Mesocricetus auratus, Peromyscus leucopus, P. maniculatus and Mus musculus] are compared, as freerunning systems in constant darkness. In analyzing their stability the distinction is made between spontaneous day-to-day instability of frequency and a longer-term lability, traceable to identified causes. The precision (day-to-day stability) of the pacemaker''s period [.tau.] is about twice as good (estimated SD = 0.6% of .hivin..tau. in Mus musculus) as the already remarkable precision of the activity rhythm it drives (average SD = 1.2% of .hivin..tau. [average pacemaker period]). Identifiable causes of long-term lability include age and several features of prior entrainment by light. The period and photoperiod of a light cycle have a predictable influence on .tau.; they cause after-effects. So do single light pulses causing a phase-shift in the freerunning system. Constant light also has an after-effect opposite in sign from the after-effect of long photoperiods. After-effects of skeleton photoperiods support the hypothesis that the transitions of light to darkness are involved in the entrainment process which leads to changes in .tau.. Both day-to-day instability and long term lability are most pronounced in species (Peromyscus maniculatus, Mus musculus) whose .**GRAPHIC**. [species average] is considerably shorter than 24 h; they are least pronounced in hamsters whose .**GRAPHIC**. is indistinguishably close to 24 h. The differences between the species in .tau. and its lability are paralleled by differences in pacemaker lability as measured in light-induced after-effects and in the extent of changes with age. The species evidently differ in the tightness with which .tau. is homeostatically conserved.