Brain research. Developmental brain researchFeb 6, 2004

Development of daily rhythms and light response in parts of the rat’s internal clock

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 4, 2026

Abstract

In 10-day-old rats, circadian rhythmicity in the dorsomedial suprachiasmatic nucleus (dm-SCN) is well expressed, while the ventrolateral suprachiasmatic nucleus (vl-SCN) shows delayed rhythmicity.

Spontaneous c-Fos activity in the dm-SCN is evident in 3- and 10-day-old rats, indicating early rhythmicity.
The vl-SCN does not show a photoperiod response until 10 days of age, with 3-day-old rats exhibiting high c-Fos activity in response to light at any time.
In 10-day-old pups, the duration of insensitivity to light in the vl-SCN is shorter under a short photoperiod (LD 8:16) compared to a long photoperiod (LD 16:8).
The difference in gate duration between photoperiods in 10-day-old pups is not yet comparable to that of adult rats.
Circadian rhythmicity may develop sooner in the dm-SCN than in the vl-SCN, while photoperiodic responses may emerge earlier in the vl-SCN.

Simplified

To ascertain whether the circadian rhythmicity of the ventrolateral (vl) suprachiasmatic nucleus (SCN) develops concurrently with that of the dorsomedial (dm) SCN and when the rhythmicity starts to respond to day length, i.e., to the photoperiod, rats with their offspring were maintained under either a long photoperiod with 16 h of light and 8 h of darkness per day (LD 16:8) or under a short, LD 8:16 photoperiod. The rhythms of spontaneous c-Fos immunoreactivity in the dm-SCN and of the light-induced c-Fos immunoreactivity in the vl-SCN were studied in the pups. In 3- and 10-day-old rats, the dm-SCN rhythm in spontaneous c-Fos immunoreactivty was already well expressed but a response to a photoperiod similar to that in adult rats has not yet been developed. The vl-SCN gate for insensitivity of c-Fos production to light at certain times was detected in 10-day but not yet in 3-day-old rats: in the latter, light exposure at any daytime induced high c-Fos immunoreactivity. In the 10-day-old pups, similarly as with adult rats, the gate was shorter under LD 8:16 than under LD 16:8, but the difference in the gate duration between the short and the long photoperiod did not yet attain that of adult animals. The data indicate that the circadian rhythmicity may develop sooner in the dm-SCN, than in the vl-SCN, whereas the photoperiodic response may develop sooner in the vl-SCN.

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Development of daily rhythms and light response in parts of the rat’s internal clock

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