Circadian pacemaker dynamics differ between diurnal and nocturnal mammals

Area of Science:

• Chronobiology
• Neuroscience
• Mammalian Physiology

Background:

• The suprachiasmatic nucleus (SCN) is the central circadian pacemaker in mammals.
• Standard measures suggest functional equivalence of the SCN across diurnal and nocturnal species.
• Coarse readouts do not fully assess SCN resetability or spatial timing coordination.

Purpose of the Study:

• To compare SCN network dynamics between a nocturnal mouse (Mus musculus) and a diurnal grass mouse (Rhabdomys pumilio).
• To investigate species-specific differences in SCN molecular clock period, phase relationships, and resetting responses.
• To analyze spatial coordination of SCN activity using calcium phase mapping.

Main Methods:

• Long-duration ex vivo recordings of SCN tissue.
• Optogenetic perturbation to stimulate the SCN.
• Single-cell calcium imaging for phase mapping of SCN activity.

Main Results:

• Rhabdomys SCN exhibited a longer intrinsic period and different phase relationship to daily stimulation compared to Mus.
• Direct SCN stimulation yielded species-specific resetting, with a notable mid-subjective-day delay in Rhabdomys.
• Calcium phase mapping revealed a graded dorsomedial-to-ventrolateral progression in Rhabdomys versus a sharper transition in Mus.

Conclusions:

• Coarse similarities in SCN structure do not guarantee conserved pacemaker dynamics across mammalian species.
• Significant species-specific differences exist in SCN network properties and responses.
• Circadian pacemaker function is more diverse than previously assumed based on broad measures.