Abstract
The suprachiasmatic nucleus (SCN) is the master circadian clock in mammals and is composed of thousands of neuronal oscillators expressing different intrinsic periods. These oscillators form a coupled network with a free-running period around 24 h in constant darkness and entrainable to the external light-dark cycle (T cycle). Coupling plays an important role in setting the period of the network and its range of entrainment. Experiments in rats have shown that two subgroups of oscillators within the SCN, a ventrolateral (VL) subgroup that receives photic input and a dorsomedial (DM) subgroup that is coupled to VL, can be desynchronized under a short (22-h) T cycle, with VL entrained to the cycle and DM free-running. We use a modified Goodwin model to understand how entrainment of the subgroups to short (22-h) and long (26-h) T cycles is influenced by light intensity, the proportion of neurons that receives photic input, and coupling heterogeneity. We find that the model’s critical value for the proportion of photically-sensitive neurons is in accord with actual experimental estimates, while the model’s inclusion of dispersed coupling can account for the experimental observation that VL and DM desynchronize more readily under the 22-h than under the 26-h T cycle. Heterogeneous intercellular coupling within the SCN is likely central to the generation of complex behavioral patterns.
Highlights
Circadian (,24 h) rhythms in physiological and behavioral measures are universal in living things, reflecting the period of the earth’s rotation
The suprachiasmatic nucleus (SCN) network is heterogeneous [2,3,4]. It can be divided into distinct functional subgroups, including a ventrolateral part (VL), which receives photic input from the retina, and a dorsomedial part (DM), which is coupled to VL; both VL and DM contribute to the generation of overt circadian rhythms in physiological and behavioral measures
It has been shown that the circadian oscillation between VL and DM can desynchronize with exposure to short T cycles [7] or after a phase shift of the light-dark cycle [6,8,9]; the VL appears to set the final phase of the SCN after the phase shift [6,8,9]
Summary
Circadian (,24 h) rhythms in physiological and behavioral measures are universal in living things, reflecting the period of the earth’s rotation. Circadian rhythms are regulated by a master clock in the suprachiasmatic nucleus (SCN) of the hypothalamus, composed of approximately 20,000 neuronal oscillators; SCN neurons are nonidentical, express different intrinsic periods, and are coupled together to form a network with a coherent output [1]. The SCN network is heterogeneous [2,3,4] It can be divided into distinct functional subgroups, including a ventrolateral part (VL), which receives photic input from the retina, and a dorsomedial part (DM), which is coupled to VL; both VL and DM contribute to the generation of overt circadian rhythms in physiological and behavioral measures. It has been shown that the circadian oscillation between VL and DM can desynchronize with exposure to short T cycles [7] or after a phase shift of the light-dark cycle [6,8,9]; the VL appears to set the final phase of the SCN after the phase shift [6,8,9]
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