Unilateral lesions of the hamster suprachiasmatic nuclei: evidence for redundant control of circadian rhythms

Summary

Several properties of vertebrate circadian rhythms can be attributed to the behavior of an underlying pacemaker system which is composed of two separate but mutually interacting circadian oscillators. As originally formulated, the model for such a pacemaker system proposed that two oscillators or populations of oscillators have different properties, specifically in their responses to light (Pittendrigh 1974; Pittendrigh and Daan 1976b). We have tested the proposition that the right and left suprachiasmatic nuclei (SCN) of the golden hamster contribute in different ways to the regulation of circadian rhythmicity by measuring the wheel-running activity rhythms of hamsters with lesions to either the right or left SCN. Although effects of unilateral or other partial SCN lesions on pacemaker properties were observed, these effects were not different in hamsters receiving right- or left-side lesions. More specifically: (1) free-running period (τ) in constant light was shorter in lesioned hamsters irrespective of the side lesioned (Fig. 3a), and the total amount of SCN destruction was found to correlate with τ (Fig. 4). (2) Phase-angle difference (Ψ) of some lesioned hamsters (both right- and left-side) during entrainment to L∶D, 14∶10 was significantly more positive than that of controls (Fig. 3b). (3) The rate of phase-shift following a shift of the light/dark cycle was not different in hamsters with right- or left-side lesions (Fig. 3c). And (4) the simultaneous expression of different circadian periods, similar to ‘splitting’, was observed in hamsters with unilateral lesions (Fig. 5). It is concluded that the right and left SCN are similar in their contributions to the control of circadian rhythmicity and that there is as yet no evidence for the permanent loss of multioscillator properties resulting from the destruction of only one of the two SCN.