Age-Related Cognitive and Volumetric Changes in the Brain of African Grasscutter (Thryonomys swinderianus (Temminck, 1827))

Abstract

The African grasscutter (AGC) (Thryonomys swinderianus) is the second largest rodent in sub-Saharan Africa. It is bred for its organoleptic and culinary properties but also serves as a research model. The aim of this study was to investigate the relationship between age-related changes in brain weight, brain volume, and spatial and nonspatial memory performance in the AGC. A total of forty-two (42) captive-derived AGCs were divided into seven age groups: 6 neonates (6 days), 6 juveniles (1 month), 6 pubertals (3 months), 6 subadults (8 months), 6 young adults (2 years), 6 middle adults (4 years), and 6 old adults (5 years). The subjects were given a series of tests to assess their spatial memory (location test) and their nonspatial memory (object recognition test). Brain samples were then collected using basic neuroanatomical techniques. The weight and volume of the brain samples were determined and their encephalization quotient (EQ) was also calculated. The results showed that spatial and nonspatial memory in AGC develops into adulthood and then declines with age. Mean brain weight increased from neonates to mid-adulthood (5.20 ± 0.31 g-11.76 ± 0.23 g) and decreased in older AGC (11.75 ± 0.22 g). In contrast, the mean weight of the olfactory bulb (OB) increased from neonates to young adults (0.28 ± 0.02 g-0.80 ± 0.03 g) and the cerebellum increased from neonates to older (0.59 ± 0 0.01 g-1.86 ± 0.06 g). Finally, the EQ decreased with age (0.73 ± 0.05-0.29 ± 0.01). Mean brain volume increased with age from neonates to young adult (5 mL-11.25 mL). Conversely, the mean volume of the OB and cerebellum increases from neonates to older age (0.33 ± 0.03 mL-0.95 ± 0.04 mL). This study shows that spatial and nonspatial memory declines during the aging process in AGC. Neonates and juveniles have higher cognitive abilities than adults and older AGC. The weight of the brain, OB, and cerebellum increases from neonates to adult and decreases slightly from middle to old adults. However, the volume of the OB and cerebellum increases with age.

Keywords: African grasscutter; aging; brain; cognition; volume; weight.

Figure 1

Investigation time of the moved object in aging AGC. 6D: 6 days, 1M: 1 month, 3M: 3 months, 8M: 8 months, 2Y: years, 4Y: 4 years, and 5Y: 5 years. Averages with different letters above the bars show a significant difference (p < 0.05) between ages. Those with the same letters show no significant difference (p > 0.05) between ages. The percentage of investigation time increased between 6 days and 2 years followed by a decrease between 2 years and 5 years.

Figure 2

Investigation time of the new object in aging AGC. 6D: 6 days, 1M: 1 month, 3M: 3 months, 8M: 8 months, 2Y: years, 4Y: 4 years, and 5Y: 5 years. Averages with different letters above the bars show a significant difference (p < 0.05) between ages. Those with the same letters show no significant difference (p > 0.05) between ages. The percentage of investigation time increased between 6 days and 2 years followed by a decrease between 2 years and 5 years.

Figure 3

Moved object discrimination index in aging AGC. 6D: 6 days, 1M: 1 month, 3M: 3 months, 8M: 8 months, 2Y: years, 4Y: 4 years, and 5Y: 5 years. Averages with different letters above the bars show a significant difference (p < 0.05) between ages. Those with the same letters show no significant difference (p > 0.05) between ages. The index of discrimination of the moved object increased between 6 days and 2 years followed by a decrease between 2 years and 5 years.

Figure 4

New object discrimination index in aging AGC. 6D: 6 days, 1M: 1 month, 3M: 3 months, 8M: 8 months, 2Y: years, 4Y: 4 years, and 5Y: 5 years. Averages with different letters above the bars show a significant difference (p < 0.05) between ages. Those with the same letters show no significant difference (p > 0.05) between ages. The discrimination index increased between 6 days and 2 years followed by a decrease between 2 years and 5 years.