Effects of light and melatonin treatment on body temperature and melatonin secretion daily rhythms in a diurnal rodent, the fat sand rat

Many mammals display predictable daily rhythmicity in both neuroendocrine function and behavior. The basic rest-activity cycles are usually consistent for a given species and vary from night-active (nocturnal), those mostly active at dawn and dusk (i.e., crepuscular), and to day-active (diurnal) species. A number of daily rhythms are oppositely phased with respect to the lightdark (LD) cycle in diurnal compared with nocturnal mammals, whereas others are equally phased with respect to the LD cycle, regardless of diurnalitynocturnality. Pineal produced melatonin (MLT) perfectly matches this phase-locked feature in that its production and secretion always occurs during the night in both diurnal and nocturnal mammals. As most rodents studied to date in the field of chronobiology are nocturnal, the aim in this study was to evaluate the effect of light manipulations and different photoperiods on a diurnal rodent, the fat sand rat, Psammomys obesus. The authors studied its daily rhythms of body temperature (Tb) and 6-sulphatoxymelatonin (6-SMT) under various photoperiodic regimes and light manipulations (acute and chronic exposures) while maintaining a constant ambient temperature of 30°C±1°C. The following protocols were used: (A) Control (CON) conditions 12L:12D; (A1) exposure to one light interference (LI) of CON-acclimated individuals for 30min, 5h after lights-off; (A2) short photoperiod (SP) acclimation (8L:16D) for 3 wks; (A3) 3 wks of SP acclimation with chronic LI of 15min, three times a night at 4-h intervals; (A4) chronic exposure to constant dim blue light (470nm, 30 lux) for 24h for 3 wks (LL). (B) The response to exogenous MLT administration, provided in drinking water, was measured under the following protocols: (B1) After chronic exposure to SP with LI, MLT was provided once, starting 1h before the end of photophase; (B2) after a continuous exposure to dim blue light, MLT was provided at 15:00h for 2h for 2 wks; (B3) to CON animals, MLT was given intraperitoneally (i.p.) at 14:00h. The results demonstrate that under CON acclimation, Psammomys obesus has robust Tb and 6-SMT daily rhythms in which the acrophase (peak time) of Tb is during the photophase, whereas that of 6-SMT is during scotophase. LI resulted in an elevation of Tb and a reduction of 6-SMT levels. A significant difference in the response was noted between acute and chronic exposure to LI, particularly in 6-SMT levels, which were lower than CON after LI and higher after chronic LI, implying an acclimation process. Constant exposure to blue light abolished Tb and 6-SMT rhythms in all the animals. MLT administration resumed the Tb daily rhythm in these animals, and had a recovery effect on the chronic LI-exposed animals, resulting in a Tb decrease. Altogether, the authors show in this study the different modifications of Tb rhythms and MLT levels in response to environmental light manipulations. These series of experiments may serve as a basis for establishing P. obesus as an animal model for further studies in chronobiology.