Light exposure impairs hematological characteristics and oxidative stress markers in male African giant pouched rat (Cricetomys gambianus) reared in captivity

African giant pouched rat (AGPR) is widely used both to conduct research projects and for meat consumption. However, being a nocturnal animal, light exposure requires a specific attention in order to improve their rearing environment. The study aimed to appreciate the effects of light exposure on hematological and oxidative stress parameters in AGPR bred in captivity. Twenty adult males AGPR weighing 945 ± 171 g were randomly allocated into four photoperiod regimes: 0L/24D, 12L/12D, 18L/06D, and 24L/0D. Housed singly during 56 days, animals received food and water ad libitum. At the end of the trial, blood and serum were collected for studied parameters analysis. The study revealed a significant (p < 0.05) increase of the number of red blood cell (RBC), white blood cell (WBC), hemoglobin (HGB), and hematocrit (HCT) in animals kept in the darkness condition compared to those reared under extended photoperiods. Except for malondialdehyde (MDA) level which was not affected by photoperiod, catalase (CAT), and superoxide dismutase (SOD) activity increased significantly (p < 0.05) for any light exposure up to 12 h per day. In conclusion, photoperiod affects the most hematological characteristics measured and has stressful effects in AGPR. This new knowledge provides a relevant information for the rearing environment of African giant pouched rat.


Introduction
African giant pouched rat (AGPR) is overexploited and widely consumed for its sweet taste and, thus, constitutes an alternative meat source for low-income populations (Oyarekua and Ketiku 2010;Ojo et al. 2016;Kenfack et al. 2020).Currently, more than one hundred AGPR are used per year as a genetic resource in several studies due to its physiological characteristics (Malekani et al. 2002;Fonou et al. 2020) close to those of other laboratory animals like mice and albino rat (Wolk 1985;Igbokwe et al. 2017).In the field of veterinary sciences, a small animal is becoming more requested.So, it is important to enhance the availability and accessibility of AGPR resources both to conduct research projects efficiently and for meat consumption despite the difficulties to rear them in captivity.Indeed, AGPR is nocturnal and fearful animal; therefore, in captive environment, light/dark cycle due to Earth rotation causes a stress affecting its physiological functions (respiration, digestion, blood system, etc.) and their breeding performances.
Stress can be defined as an imbalance between the production of reactive oxygen species and the activity of antioxidant system of the body (Luczaj et al. 2017).In animals, biomarkers such as catalase, superoxide dismutase, lipid peroxidase or malondialdehyde, glutathione peroxidase, and reduced glutathione serve as stress sign post (Aitken and Roman 2008;Choudhary et al. 2018).However, recent studies showed that hematological parameters appear as secondary stress indicators (Abbas et al. 2008;Al-Emran 2017;Thrall et al. 2002).The blood parameters currently assessed include the red blood cell (RBC) counts, hemoglobin concentration (Hbc), packed cell volume (PCV), mean corpuscular values, white blood cell counts (WBC), corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) (Igbokwe et al. 2017;Al-Emran 2017;Thrall et al. 2002).Nevertheless, both blood and oxidative stress parameters are affected by environmental factors like temperature, food restriction, and photoperiod (Al-Emran 2017; Bani et al. 2009).Indeed, (Bani et al. 2009) observed a significant effect of photoperiod on Hb and erythrocyte values in young beluga sturgeon.Furthermore, (Olanrewaju et al. 2006) reported a significant increase of MCHC and Hb in broiler submitted to short photoperiod.They also observed that darkness exposure is less stressful in broiler chicken than long light exposure.(Choudhary et al. 2018) recorded an enhancement of lipid peroxidation and reduction of catalase and superoxide dismutase activities in ewe exposed to long light.If the effects of photoperiod are known in conventional animal species, it remains unawarded in non-conventional species such as AGPR.

Study site
The study was carried out at the Teaching and Research Farm (TRF) of the Department of Animal Science, University of Dschang (LN 5 to 7°, LE 8 to 12°).TRF is located in the Western Sudano Guinean zone of Cameroon at an altitude of 1500 m above sea level.The mean annual temperature and relative humidity are 20 °C and 60%, respectively.The annual rainfall varies between 1900 and 2010 mm.The rainy season goes from mid-March to mid-November and the dry season from mid-November to mid-March.
Except photoperiod, they were kept under the same hygienic and managerial conditions.
Experimental protocols used in this study were approved by Ethical Committee of the Department of Animal Sciences (University of Dschang) and were in accordance with the guidelines for the care and use of laboratory animals.For these reasons, strategies to reduce the number of animals used and minimize their suffering during the experiment were applied.

Assessment of oxidative stress markers
Animals were anesthetized with ether vapor then killed, and blood was collected from jugular vein using dry and EDTA tubes.Blood in the dry tube was centrifuged at 3000 rpm for 15 min, and the serum obtained served to evaluate oxidative stress markers.Thiobarbituric acid method described by (Nilsson et al. 1989) allowed to assess the concentration of malondialdehyde (MDA), while superoxide dismutase (SOD) activity was determined according to (Misra and Fridovich 1972).The catalase (CAT) activity was assessed using the chromic acetate method as described by (Sinha 1972).Spectrophotometer (model UV-1900, Shanghai) served to read the various optical densities.

Statistical analysis
Data obtained were prior submitted to the assumption of normality and then using one-way ANOVA; effects of photoperiod on studied parameters were assessed.Duncan's test allowed to separate the means with a p-value < 0.05.Results were expressed as means ± standard deviation.SPSS IBM statistics software 20.0 was used to perform the analyses.

Results
In African giant pouched rat, the activity of catalase and superoxide dismutase increased significantly (p < 0.05) for any light exposure up to 12 h per day (Figs. 1 and 2).However, the reduction of light exposure did not affect the level of malondialdehyde (MDA) in African giant pouched rat.But, it displayed a downward trend in animals receiving 18 and 24 h of light per day (Fig. 3).
Blood parameters showed a significant (p < 0.05) increase of the number of red blood cell (RBC), white blood cell (WBC), hemoglobin (HGB), and hematocrit (HCT) in animals kept in the darkened room compared to those reared under extended photoperiods.However, the different photoperiods had no significant effects on the levels of mean corpuscular volume (MCV), plateletocrit (PCT), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) (Table 1).

Discussion
Due to its ability to affect physiological parameters in several animals, photoperiod constitutes a crucial breeding factor requesting a particular attention (Al-Emran 2017; Bani et al. 2009;Olanrewaju et al. 2006).In this study, results revealed that animals subjected to short photoperiod recorded a significant decrease of oxidative stress markers (catalase and superoxide dismutase) which is in disagreement with (Chenyu et al. 2019) who found a significant increase of CAT and SOD in goat reared under short photoperiods.Two potential assumptions could be stated to explain the low activity of catalase and superoxide dismutase in animals exposed to reduced photoperiod.Firstly, the effect of photoperiod varies with respect to sex and specie (Shinomiya et al. 2014).AGPR is a nocturnal animal, thereby rear them under short photoperiod would mimic their wildlife conditions and was not therefore stressed enough.Secondly, melatonin is a powerful direct free radical scavenger greatly secreted during photoperiod decreasing (Choudhary et al. 2018;Tamura et al. 2008).Melatonin is able to neutralize reactive oxygen species including superoxide radical (O 2-), hydroxyl radical (•OH), hydrogen peroxide (H 2 O 2 ), and peroxynitrite anion (Tamura et al. 2008).
In African giant pouched rat, the number of red blood cell (RBC) varies according to sex.Indeed, it is higher in female than in male (Igbokwe et al. 2017).In this study, without any consideration to various photoperiod applied, the number of RBC was low compared to reference value previously published by (Igbokwe et al. 2017) in juvenile rat (Cricetomys gambianus) suggesting a negative correlation between the number of RBC and age.
In the framework of this study, the number of red blood cell and hemoglobin decreased in animals subjected to prolonged light periods compared with those maintained in the darkness conditions due to stress.This result is in agreement with (Al-Emran 2017) who showed a significant decreased of red blood cell in fish subjected to extended photoperiods, but was in disagreement with (Ghomi et al. 2011) who observed that photoperiod variation did not affect blood parameters.
The assessment of hematological parameters also revealed a significant increase of WBC in darkened animals compared to that of animals submitted to continuous light.This finding is similar with (Zheng et al. 2013) who observed a significant increase of white blood cells in broiler chicken kept in darkness.This enhancement of white blood cells in animals kept in darkness could be due to the action of melatonin on the leukocyte production process inside bone marrow cells.Indeed, as prior mentioned melatonin is greatly released during darkness phases, it displays several actions including leukocyte production (Dalal 2016).In conclusion, the findings indicate that extended photoperiods (above of 12 h/day) affect most hematological characteristics and has stressful effects.This new knowledge is a relevant information for the rearing environment of African giant pouched rat.

Data availability
The datasets analyzed during the current study are available from the corresponding authors on reasonable request.

Fig. 1
Fig. 1 Effect of photoperiod on serum level of Catalase in male African giant pouched rat.a,b : bar affected with the same letter does not differ significantly (p > 0.05)

Fig. 2
Fig. 2 Effect of photoperiod on serum level of superoxide dismutase in male African giant pouched rat.a,b : bar affected with the same letter does not differ significantly (p > 0.05)

Table 1
Effect of photoperiod on the hematological parameters of male African giant pouched rat p probability, WBC white blood cell, RBC red blood cell, HGB hemoglobin, HCT hematocrit, PLT platelets; MCV mean corpuscular volume, PCT plateletocrit, MCH mean corpuscular hemoglobin, MCHC mean corpuscular hemoglobin concentration a, b: on the same line values affected with different letters differ significantly (p < 0.05)