Maternal separation in the light or dark phase of the circadian cycle has different effects on the corticosterone levels and anxiety-like behavior in male adult rats

Maternal care in early life is essential for the growth and development of an offspring. The light phase of the circadian cycle is the dam's most caring phase for her pups. Therefore, we hypothesized that, in the long term, maternal separation (an early-life stress model) in the light phase can be more deleterious than maternal separation in the dark phase. This study aims to compare the effects of maternal separation in the dark or light phase of the circadian cycle on the behavioral and physiological parameters of adult male Wistar rats. From the first to the fourteenth day of life, litters were separated from their mothers for six hours, forming the following experimental groups: Control (C), Maternal Separation in the Light (MSL), and Maternal Separation in the Dark (MSD). After weaning, these groups were left undisturbed until the start of behavioral tests in adulthood. Behavioral measures (standard diet intake, palatable diet intake, and anxiety-like behavior in the elevated plus maze), as well as retroperitoneal adipose tissue weight, and corticosterone levels in response to feeding stress, were assessed between the ages of 120 and 180 days. The results showed that, in adulthood, the food intake after an eight-hour fast or feeding stress had not changed. Anxiety behavior was analyzed by measuring time in the open or closed arms of the elevated plus-maze. The corticosterone levels were measured after food stress, and quantified by using the enzyme-linked immunosorbent assay (ELISA) method. The retroperitoneal adipose tissue weight was also measured. The MSD group presented higher corticosterone levels (MSD = 173.9 ± 12.7) when compared to the C (4.21 ± 0.3) and MSL (4.25 ± 0.28, p < 0.001) groups. In the anxiety test, we observed that the duration in the open arm was shorter for the MSL group (27.4 ± 3.7) compared to the control (112.9 ± 9.6, p < 00.1). There was no change in the MSD group compared to the control, with regard to the time spent in the open arm of the maze. An effect of maternal separation on the consumption of a palatable diet was observed. There was an increase in the palatable diet intake in the MSL (16.3 ± 1.9) and MSD (15.7 ± 0.5) groups compared to the control (11.4 ± 1.2, p < 0.05). In response to stress and food deprivation, there was no difference between groups in food consumption. The adipose tissue weight was higher in the MSD group (7.12 ± 0.29) compared to the control (4.21 ± 0.3) and MSL (4.25 ± 0.28, p < 0.001) groups. Based on the results observed, we concluded that the light-phase stress is more detrimental to emotional behavior during the first two weeks of life, and the dark-phase stress is more detrimental to Hypothalamus-Pituitary-Adrenal (HPA)-axis functioning. Furthermore, the higher intake of a palatable diet could be interpreted as a coping behavior to these early stress-related modifications.