Editors' ChoiceCircadian Rhythms

Cacophonous Clock

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Science Signaling  25 Mar 2014:
Vol. 7, Issue 318, pp. ec78
DOI: 10.1126/scisignal.2005297

Circadian rhythmic changes in biological function occur in response to daily changes in the intensity of sunlight. The suprachiasmatic nucleus in the hypothalamus is the central pattern generator that times the circadian clock; however, many peripheral tissues also exhibit intrinsic circadian oscillations and little is known about circadian rhythms in the auditory system (see Loudon). Meltser et al. identified that in mice, which are nocturnal, the cells of the cochlea in the inner ear had an intrinsic circadian rhythm and were more sensitive to noise-induced damage during their active period at night. The speed and amplitude of the acoustic startle response was greater in mice exposed to noise during the day than at night. Moreover, mice exposed to traumatic noise at night did not recover as well from hearing damage as those exposed during the day. Cochlear explants showed rhythmic expression of clock genes, including Per2, and a Per2 luciferase reporter (Per:luc) over 24 hours. Exposure to traumatic noise suppressed clock gene expression in cochlea to a greater extent in mice exposed at night than in the day. Day, but not night, exposure of mice to traumatic noise increased cochlear expression of the gene encoding brain-derived neurotrophic factor (Bdnf), which plays a role in cochlear synaptogenesis and binds to tropomyosin receptor kinase type B (TrkB). Exposing cochlear explants to the TrkB agonist 7,8-dihydroxyflavone (DHF) increased the amplitude, delayed the phase, and altered the period of Per:luc activity. Mice injected with DHF and exposed to traumatic noise during the night recovered from hearing damage. Thus, Bdnf-TrkB signaling protects the cochlea from noise-induced damage during the day when mice rest.

I. Meltser, C. R. Cederroth, V. Basinou, S. Savelyev, G. S. Lundkvist, B. Canlon, TrkB-mediated protection against circadian sensitivity to noise trauma in the murine cochlea. Curr. Biol. 24, 658–663 (2014). [PubMed]

A. S. I. Loudon, Hearing damage and deafness: A role for the circadian clock. Curr. Biol. 24, R232–R234 (2014). [Online Journal]