Effects of photophase illuminance on locomotor activity, urine production and urinary 6-sulfatoxymelatonin in nocturnal and diurnal South African rodents.

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 a key role in maintaining circadian rhythms in rodents, irrespective of their temporal activity rhythm.