Phase-resetting sensitivity of the suprachiasmatic nucleus and oscillator amplitude: Reply to letter by Ruby

The amplitude of a circadian oscillator influences its response to a phase-resetting stimulus. The suprachiasmatic nucleus (SCN) is unique among circadian clocks in mammals in that the network connections among its neurons confer robustness both in amplitude and in resilience to perturbations. With reduced coupling among SCN neurons, the SCN becomes more susceptible to external phase-shifting stimuli. Thus, stimuli of the same strength will elicit different responses from the same tissue under different states of internal coupling. In his letter, Ruby (2011 [this issue]) discusses potential causes for discrepancies in studies that report dissimilar responses of the SCN to temperature changes. Here, we propose that the differences are likely due to a species difference and a difference in oscillator amplitude. These differences more likely result from inherent differences between mice and rats than from experimental procedures.