Abstract:
Effects of photophase illuminance (1, 10, 100 and 330 lx of white
incandescent lighting) on daily rhythms of locomotor activity, urine
production and 6-sulfatoxymelatonin (6-SMT; 10 versus 330 lx) were
studied in nocturnal Namaqua rock mice (Micaelamys namaquensis)
and diurnal four-striped field mice (Rhabdomys pumilio). Micaelamys
namaquensis was consistently nocturnal (∼90–94% nocturnal
activity), whereas considerable individual variation marked activity
profiles in R. pumilio, but with activity mostly pronounced around
twilight (∼55–66% diurnal activity). The amplitude of daily activity
was distinctly affected by light intensity and this effect was greater in
M. namaquensis than in R. pumilio. Only M. namaquensis displayed a
distinctive daily rhythm of urine production, which correlated with
its activity rhythm. Mean daily urine production appeared to be
attenuated under dim photophase conditions, particularly in R.
pumilio. The results suggest that the circadian regulation of
locomotor activity and urine production possesses separate
sensitivity thresholds to photophase illuminance. Micaelamys
namaquensis expressed a significant daily 6-SMT rhythm that
peaked during the late night, but the rhythm was attenuated by the
brighter photophase cycle (330 lx). Rhabdomys pumilio appeared to
express an ultradian 6-SMT rhythm under both lighting regimes with
comparable mean daily 6-SMT values, but with different temporal
patterns. It is widely known that a natural dark phase which is
undisturbed by artificial light is essential for optimal circadian function.
Here, we show that light intensity during the photophase also plays atheir temporal activity rhythm.
key role in maintaining circadian rhythms in rodents, irrespective of
