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. 2021 Apr;36(2):196-199.
doi: 10.1177/0748730420972817. Epub 2020 Nov 25.

Entrainment Is NOT Synchronization: An Important Distinction and Its Implications

Eric L Bittman  1
Affiliations

Entrainment Is NOT Synchronization: An Important Distinction and Its Implications

Eric L Bittman. J Biol Rhythms. 2021 Apr.
No abstract available

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Conflict of interest statement

CONFLICT OF INTEREST STATEMENT

The author has no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.
Figure 1.
Locomotor activity rhythms can be synchronized with the dark phase of the L:D cycle in animals lacking a functional circadian clock. (a) Conditional deletion of Bmal1 in cells expressing the vesicular inhibitory amino acid transporter Vgat, including γ-aminobutyric acid-containing neurons of the suprachiasmatic nuclei, results in mice that avoid light, showing a masked activity rhythm. (b) A control mouse is also nocturnal in the L:D cycle, but free runs from the entrained phase when transferred to constant darkness. Modified from Weaver et al. (2018).
Figure 2.
Figure 2.
Entrained oscillations differ from rhythms that are merely synchronized with the environmental cycle. (a) Unlike rhythmic activity patterns that are synchronized by the light:dark cycle, entrained locomotor activity varies in phase as the period of the zeitgeber cycle (T) changes and breaks entrainment when the difference between T and the endogenous period (τ) exceeds a limit. Shaded regions of this double-plotted actogram indicate darkness; the duration of the light and dark phases remained constant as T was gradually shortened from 24.1 h to 20 h. Running onset in this wild-type hamster was approximately synchronous with the onset of darkness for the first week of the experiment, but the phase angle (ψ) became increasingly negative as shortening of T required progressively greater phase advances. On day 26, the difference between T and τ exceeded the maximum possible phase shift as predicted by the amplitude of the phase response curve. From this point on, the hamster exhibited relative coordination. (b) Wheel running activity is plotted modulo τ in a short-period double mutant (super duper) hamster whose free running period is about 18 h and which shows a high-amplitude (Type 0) phase response curve. Activity onset is approximately synchronous with dark onset on a 3L:16.5D cycle (T = 19.5, well below the limit of entrainment for a wild-type). This animal must adopt such a phase angle to entrain by phase delays. When released into constant darkness (DD) on cycle 19, the free run departs from the entrained phase, indicating that the light cycle has, in Aschoff’s phrase, “caught the clock.” (c) When the same super duper hamster was exposed to a 3L:12.6D (T = 15.6) cycle, activity offset was approximately synchronous with lights on. T is now shorter than τ, so the hamster adopts a phase angle ψ that enables it to entrain by phase advances. When released into DD, the free run again departs from the entrained phase. In contrast, masking may lead to synchrony but entails neither a change of ψ with T nor control of phase of the endogenous clock by the environmental cycle. Figure modified from Bittman (2014).
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